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#1 2010-12-17 05:26:52

sdritchey
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Registered: 2010-12-17
Posts: 20

6550 Drop-In Compatible Tubes?

I have an early 70's Ampeg V4 head (w/o the master volume) that was completely renovated a few years ago.  At that time I was primarily using the amp for bass guitar, and the tech recommended converting the amp to run 6550's, which was done.  It sounds awesome, and for bass I love the 6550's.

Now I am playing guitar in a new band, and the amp is just too loud.  Even running it with 2 tubes removed (and the ohm switch set accordingly), I still have to turn it up too loud to get that nice breakup, though it's better than with the 4.  I have read about various tubes breaking up at a lower volume (like the original 7027's), but as my amp is modified, I can't just drop in the old tubes right?  I have also heard that the 7027's color the sound more than the 6550's, and for me this would be desirable while playing guitar (but not necessarily playing bass).  I still play bass with the amp, and would love to have say 4 x 6550's for bass, and 2 x ? for guitar.

So my question is:  What tubes can I experiment with that are drop-in compatible with my setup?  And also, if my setup is not ideal for switching between bass and guitar, how might I modify the amp to better serve this dual role?

Thanks in advance for all advice!

- Steve

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#2 2010-12-17 18:52:24

rob
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Registered: 2008-10-28
Posts: 171

Re: 6550 Drop-In Compatible Tubes?

Unfortunatly you can't just drop in a a different tube type without re-setting the bias. I do not think that changing tube types will significantly reduce the v's fury. These amps put out about 60watts with 2 tubes. This is going to be too loud for guitar for most venues without an attenuator. For guitar I like to set the volume at around 1:00. I have to use a THD hotplate to do this.

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#3 2010-12-17 19:31:27

sdritchey
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Registered: 2010-12-17
Posts: 20

Re: 6550 Drop-In Compatible Tubes?

I'm def considering an attenuater, but I would like to experiment with some tubes first to fine tune my sound.  I'm not 100% pleased with the 6550 for guitar.  What I'm looking for is an earlier / smoother breakup, and a bit more color to the sound, though I'm aware I might be in the wrong tube family to get those characteristics.

My other amp is self-biasing, so I can just drop in a big range of tubes.  Is it possible to modify the V4 for self-biasing?  Or is there some other mod that enables easy rebiasing between tube types?  I have read about an external bias switch, what's that?

Staying in my current tube family, what about KT88's?  I have read conflicting info that they are drop-in replacements for 6550, and are better for guitar.  Do I just need to take the amp to my tech and ensure that the bias is set so that either one will work?  Anyone try KT88's in a V4?

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#4 2010-12-18 06:56:31

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 6550 Drop-In Compatible Tubes?

The KT88 sounds every bit as big as the 6550 and more.

if you don't like the 6550, the kt88 is even further in that same direction.  Very big and solid.

if you want more color... go back to 7027/6L6GC. 

If you want more breakup at a lower volume,  you are playing the wrong amp.    Plain and simple.

I know that isn't the answer you want to hear.  But it is the answer.

I have heard a V2 that was modified for cathode bias,  and it was marginally quieter than it was when at fixed bias. we removed the mod and put it back to original spec. 

There are SOOOOO many great low watt amps,  ampeg made dozens of excellent 16-30 watt combos.  as did fender.
if you want something more modern with some more gain try an orange ad30 or rockerverb 30.

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#5 2010-12-18 17:13:48

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: 6550 Drop-In Compatible Tubes?

Yeah, don't screw with power amp overhaul mods.  Attenuator, or a smaller amp.  IF you want breakup for guitar but don't like 6550s (??), try El34s. But if you don't like how it breaks up, AND you want it quieter, what is it you like about the amp for guitar?  Keep it for bass and get another amp for guitar. It's a big loud amp.  It's equally silly as trying to make it a twin into a small dirty amp... 

Plenty of good little Ampeg amps that breakup earlier. If you're broke, save, or buy a used pedal... 

In my experience, Even my ~6 watt silverface champ with corrected bias - once breaking up, is friggin' loud.  Try a carr mercury and see what I mean.

Last edited by Liquids (2010-12-18 17:15:22)


Matthew

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#6 2010-12-18 20:45:09

sdritchey
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Registered: 2010-12-17
Posts: 20

Re: 6550 Drop-In Compatible Tubes?

Thanks for the replies guys, really clarifies the limitations, and that is what I'm looking for.

I'm not looking for a lot of breakup. We are a garage, blues inspired indie band.  I love the V4's big spacious sound when clean, I just want a little of that breakup edge at a slightly lower volume.  I can afford a different amp and will go that route if need be (or attenuator), but I love this one and want to see if it will work for me in this role first.  I'm also learning a ton about my amp, so thanks for replying to my noob questions!

For guitar I think I would prefer the 7027/6L6GC tubes.  Sounds like the bias mods aren't a good idea.  Is it possible to bias in such a way that the 2 sets of tubes (4x6550 & 2x6L6GC) can be switched out without a rebias?  Is there a middle ground or is it one or the other?

One other idea, what about preamp tubes?  From what I've read, changing the 12AX7's for 5751's or 12AY7's would have the effect of lowering the volume, but would give me more clean headroom (which I'm not looking for) right?  There's again conflicting info out there on that.

Liquids, actually my other amp is a champ copy, a 5w Kendrick 118.  I LOVE the sound of that baby and it is loud, but it doesn't hold up in a band unless it's mic'd.  It does sound sweet, and it does have a line out, maybe I could use it as my distortion 'pedal'?

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#7 2010-12-18 22:37:28

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: 6550 Drop-In Compatible Tubes?

I think a good attenuator is definitely what you want if you like the way the ampeg breaks up, and probably your best option.  But for one, make sure you set the sensitivity to it's loudest setting to get maximum potential preamp breakup happening.

Nothing else will get the V4 sound but the Ampeg, if you like that sound.  Changing preamp tubes will not get you sooner breakup, or if so, it may be sort of strange.  Maybe a 12AT7 in the phase inverter position would be worth a try?  Few tubes have more gain than a 12ax7, the one's you listed have less gain.   Easy enough to try a hotter tube in the phase inverter, since it's not a mod.  But unless your already close to breakup volume, it may not be that significant, or sound bad...not sure.

Do you know how your amp is currently biased?  If it still has the stock 75k resistor in there, those 6550s may be biased REALLY cold and while it will still be loud and proud, a good bias may make the breakup sooner and/or nicer.  I really liked the way the 6550s sounded breaking up, personally, far more than 6L6/7027 types.  Clean too.  But to each their own.  Sounded like a huge & fat EL34 without that high mid thing, more even, but similar 'classy' crunch.

Anyhow, no, you can't just swamp in 6L6GCs in without a rebias here, unless your amp has the stock 75k bias resistor, which will be perfect for any good set of 6L6GCs you throw in there that can take the extra high voltage (SED).  That resistor would be fine for most 6L6s but probably less than ideal for 6550s - really, really, really cold in fact.  But that may be to your advantage, in this case, if you just want to plug and play.  But again, so could getting those 6550s closer to 60% via a bias....nice to have friends who can do this stuff for you while you try it out, isnt it?  big_smile

If you only want a hint of bluesy dirt, a tweed (run about 300v on the plates) 1x8 champ is not the deal, agreed.  It says 5 watts clean(?) but I doubt that seriously, and 1x8 speakers in that tiny box just don't have the room projection to get over most drummers live even when crunching, I hear you.  On the other hand, my champ is runnin 430v, so more headroom is theoretically available than in the tweed version even though I did defeat the negative feedback.   More so, the (cathode) bias is corrected in mine for that voltage (fender/CBS just kept raisin the voltages in the '64 champ circuit) for maximum clean headroom, so it breaks up a lot later and nicer than before to the ear.   Later champs clip very asymmetrically as the plate voltage went up from the tweed champs, but the cathode bias resistor value never changed along with it!

Lots of guys get a lot of mojo out of a Princeton for what you're doing.  Look at Charlie Hunter and Jim Campilongo, and I even heard Mike Bloomfield used to use one and mic it up. Plenty of others too.  It has a unique breakup tone due to the cathodyne phase inverter (only blackface type fender that used that, I believe).   In theory a Princeton is only 3-5db louder than the average Champ, but it has a bigger speaker, a better baffle, probably better fidelity transformers.  It's still definitely  nice to have it as a head to run it through at least a 2x10 for max room coverage, etc. 

You should definitely try your champ through a 2x12 or a real guitar cab (whatever you run your ampeg through) especially if you can match impedances somehow.  It might surprise you.  Yeah, so long as you use the champ running through a speaker, you can use the line out but it may not sound as good as you think.  Maybe just a bigger speaker cab is a better idea.

If none of that does it for you, and you can buy an new amp, I'd recommend a Dr Z Max Jr. (or Senior). Lots of dynamic, bluesy tones in there, especially good ones if you like a little breakup without bleeding ears (read: Ampeg).

Lastly, I just remembered that I was explaining a concept to a friend.  If you can but a TRUE high voltage (for pedals) op-amp based clean boost pedal like the barber launch pad or MI audio boost and buff or others, which have tons of clean headroom boost available, that could be your ticket.   I say op amp since a lot of booster pedals on the market use discrete compenents that end up adding their own color and distortion to the sound - which I assume you don't want.  Op amps at high voltage are clean clean clean, the same signal only louder

The ampeg is very clean but on any tube amp, if you drive the input hard enough, it will clip the first tube stage.  I tried it the other day for a friend where I sent a really strong, clean  signal out of my effects chain, and got it to the point that it was clipping the first tube at whisper volume.  See if you like that sound. In combination with the right sensitivity setting on the V4, you won't get 'power amp' distortion, but you just may be surprised at how much you like the preamp tube stages slamming into each other.

Last edited by Liquids (2010-12-18 22:45:46)


Matthew

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#8 2010-12-19 03:30:25

sdritchey
Member
Registered: 2010-12-17
Posts: 20

Re: 6550 Drop-In Compatible Tubes?

Liquid,

Thank you for the amount of information in that reply!  Lots of great ideas to experiment with...

I can't believe I overlooked the sensitivity switch.  I played with it a bit, but as I didn't really know exactly what it did I wasn't very conscious of the potential for getting me earlier breakup.  I currently rent a 4x12 Marshall cabinet (that's all they have avail), and am going to go ahead and purchase a 2x12, as this may help my volume go down a good bit.  Once I get a handle on how that sounds, I'll look into the current biasing setup and potentially switch back to the 6L6's.  The boost pedal sounds interesting as well...

I'll report back...


Steve

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#9 2010-12-19 09:38:37

gsb
Member
Registered: 2010-02-10
Posts: 58

Re: 6550 Drop-In Compatible Tubes?

The next thing is probably using speaker cab with lower sensitivity speakers. I mean. My bandmates hav e VOX AC30 (30w) with greenback spekaers and the other on have AC120 (120w) with mcKenzie speakers. Guess what. AC30 is louder than ac120. Both are serviced so it isnt a tech failure. mckenzie speakers are way less efficient than greenbacks. We can easily drive Ac120 into power tube distortion.

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#10 2010-12-19 19:02:40

sdritchey
Member
Registered: 2010-12-17
Posts: 20

Re: 6550 Drop-In Compatible Tubes?

Yeah, I was considering Greenbacks bc of their reputed good tone, but mostly bc they are pretty inefficient.  Can't seem to find too much info on McKenzie speakers out there, are they vintage?

I've moved the cabinet discussion to: http://ampegv4.com/forum/viewtopic.php?pid=5136#p5136

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#11 2010-12-19 21:39:50

gsb
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Registered: 2010-02-10
Posts: 58

Re: 6550 Drop-In Compatible Tubes?

Greenback inefficient? whoa... I think they're louder than most of speakers on the market (incl. V30). I think they are around 100dB?

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#12 2010-12-19 22:18:21

sdritchey
Member
Registered: 2010-12-17
Posts: 20

Re: 6550 Drop-In Compatible Tubes?

The Heritage G12H is 100db, but the others are all lower...  The Heritage G12M is 96db, comparatively pretty low I think.

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#13 2010-12-20 13:04:04

rob
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Registered: 2008-10-28
Posts: 171

Re: 6550 Drop-In Compatible Tubes?

Greenbacks sound great with these amps for guitar. The beauty of using an attenuator, and not a lower powered amp is you have the headroom if you need it. Cathode bias works great with the V4. It sounds better both clean and distorted,and makes the bias more stable and reliable.

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#14 2010-12-20 15:59:33

teledeluxe
Member
From: Chicago, IL
Registered: 2008-09-24
Posts: 467

Re: 6550 Drop-In Compatible Tubes?

A Greenbacks is definitely less efficient than a V30! I personally don't like speaker breakup, and the greenback sounds extra ratty to me. I no likey. But then again I hate V30s too. If I was doing Celestion I'd go G12-65 or G12H-30. Or better yet, a combo of both.


2 wrongs don't make a right, but 3 lefts do!

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#15 2010-12-20 17:33:09

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 6550 Drop-In Compatible Tubes?

just get a hotplate/powerbrake/powersoak.

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#16 2010-12-21 10:45:47

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: 6550 Drop-In Compatible Tubes?

Josh wrote:

Why do so many people buy this amp and then try to turn it into something that it isn't?

It's pretty common all around with most amps that are likable - especially affordable ones.  It's always fun to tweak subtly to one's own taste, but the line between major and minor modification is often in the eye of the beholder...

People have been trying to get the sound of big amps breaking up at club volume for a while.  Keep in mind that a Marshall from the same year is easily 3x to 10x as expensive as a V4.


Matthew

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#17 2010-12-21 14:06:24

rob
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Registered: 2008-10-28
Posts: 171

Re: 6550 Drop-In Compatible Tubes?

I agree, but trying different output tubes, mods to the bias method/ supply and speaker selection hardly can be considerd making the amp what it isn't. These amps sound terrific and can work great for guitar in small clubs given the right set up.

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#18 2010-12-21 14:32:10

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: 6550 Drop-In Compatible Tubes?

rob wrote:

I agree, but trying different output tubes, mods to the bias method/ supply and speaker selection hardly can be considered making the amp what it isn't. These amps sound terrific and can work great for guitar in small clubs given the right set up.

I agree, but again, it's all in the eye of the beholder and the amp.  To some, trying to make a fender run El34s and or Celestions, or a Marshall run 6L6GCs or Eminence speakers is just running different output tubes and speakers.  To others, it's heresy!

Anyhow - on a more technical level, Rob, with your experience on cathode bias, what is is that makes people think cathode bias sounds better?  With a cap across the cathode resistor, the bias point should be pretty stable, if a large enough value used, no?  Or, no less 'stable' than the grid when fixed bias.

And in that case, the only difference between that and 'fixed bias' (or, more correctly, negative grid voltage biasing) is a difference of the effective plate voltage/headroom, all else being equal?

But from Aiken Amps: "Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation.  This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive."  etc.

I guess I don't get why it's functionally or sonically different.  The only thing I can see is that when signal is applied to a cathode bias amp, the cathode becomes more positive and further from the grid, making the bias 'colder' for brief periods. That is, the power stage is hottest with no signal, and colder with a signal.

This is the opposite of 'fixed' or 'negative grid bias' where the grid becomes more positive, and hence closer to the cathode in voltage as signal is applied, making it coldest with no signal, and 'hotter' as more signal is applied...

I can accept that cathode bias AB1 or single ended is for some reason less efficient than standard %70 or less fixed bias, and that it lowers the effective plate voltage and hence has less headroom.  Likewise I can see that it is more likely to clip the power section asymmetrically unless everything is 'just so.'  I can also see that for those that like a hot bias, cathode bias could be the ultimate tone and safer than hot class AB1.

But otherwise, I don't really get why it sounds different than just biasing it really hot, all else being equal!

Rob - have you ever posted gut shots of how you wired up your cathode bias mod?

I'm screwing with my silver face champ of late and found I much prefer the tone, both distorted and clean, with a more conservative idle bias than standard.  So again I find I prefer a colder bias.  But single ended is 'sort of' another level beyond AB1 cathode bias...

Last edited by Liquids (2010-12-21 14:39:54)


Matthew

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#19 2010-12-21 16:18:40

sdritchey
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Registered: 2010-12-17
Posts: 20

Re: 6550 Drop-In Compatible Tubes?

I was going to broach this same topic in a different thread in the future.  rob you seem to really favor the cathode bias sound, and have my interest piqued with the 'harmonically rich' description I've seen you post elsewhere on this site.  However hangman seemed to discourage it, but maybe he was referring only to whether or not it would quiet down the amp.  Would love more info...

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#20 2010-12-21 18:02:04

hangman
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From: Seattle Washington
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Posts: 1848

Re: 6550 Drop-In Compatible Tubes?

in past threads I don't know if I would say that I discourage doing the cathode bypass as much as I don't think its a huge difference in volume.
as for harmonically rich... I think the difference is noticeable but its not night and day.

I have noticed with the V4 more than any marshall or fender that people want it to be quieter.  and all I can say is get something else. 

I will say this and then shut up about my opinions regarding this,  I think the concept of taking a 100 watt fixed bias amp and modifying it to be a 40 watt cathode biased amp seems so backwards.   there are so many 30 watt cathode baised amps in the v4 price range that it seems silly to take one of the defining characteristics of the v4 away all so it can be another entry on a long list of good and cheap 30-40 watt amps.

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#21 2010-12-21 19:47:34

rob
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Registered: 2008-10-28
Posts: 171

Re: 6550 Drop-In Compatible Tubes?

The reason I converted to cathode bias was not to make the amp quieter, or to make it sound different per se. I play guitar primarily. I run the amp hard and use the guitar volume for distortion (no distortion pedals). I absolutly love the sound of the cranked V-4. No other amp really sounds like it. These early amps are a little finickey with the stock  bias setup (as many of us have noticed), especially when pushed hard with russian tubes. I realize there are other solutions, but cathode bias is simple, very reliable and is very safe when operated in this mode. Although its sonic differances may be considered subtle to some ears, I have had good success with it.
The reason the "70% rule" does not apply to cathode bias is the bias swing from idle to full power output is less dramatic with cathode bias due to the self regulating affect of the resistor. Thus idle current can be considerably higher without violating the tubes plate dissipation at full power. A good illustration of this is shown on the RCA tube data sheet for the 7027A. There is a slight loss of power output when compared to fixed bias(with bypass cap.), but not much.  My cathode biased V-4 with 2 2027A tubes is more powerful than my 73' 50 watt Marshall.
From my experience, for guitar, every amp I have converted to cathode bias sounds slightly better (for guitar). That is only my opinion. They appear to have better sustain both clean and distorted, with good note clarity, and slightly more compressed.

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#22 2010-12-21 20:12:57

sdritchey
Member
Registered: 2010-12-17
Posts: 20

Re: 6550 Drop-In Compatible Tubes?

I tend to agree with what others and hangman express, that the original design of an amp should be understood and respected, that at some point of modification the amp isn't what it was anymore.  Making it different is probably the point for some people.  There is however a huge grey area there.  Which part of the design is the essence?  How much is too much?

For me as a novice on the inner workings of tube amplification, it's only through forums such as this, where most of you own my exact amp, that I can ask questions, read opinions and come to my own conclusions.  This new band is really leading me down a path of experimentation to find a special sound that works for me.  I've played guitar and bass for 20 yrs, but I just haven't had the opportunity to play very many different amps, much less be aware of the internal differences and how this affects the final sound.  For instance, I would never know that as hangman says "...there are so many 30 watt cathode baised amps in the v4 price range".  I don't even know what cathode bias is!  I just know some of you disagree on it, and in researching your opinions I learn a lot.

I guess what I'm trying to say is thank you for all chiming in with your opinions and information.  I know to those of you who have been here for years, "...my V4 is too loud" is probably the most annoying question someone can ask.  But your answers are very helpful to someone like me, and catalyze new chains of thought and possibilities.

Last edited by sdritchey (2010-12-21 20:14:18)

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#23 2010-12-21 23:36:29

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 6550 Drop-In Compatible Tubes?

SDrictchey,

Thats the whole Idea I think.  There is no "correct" way to look at it. 

there are many sides to every issue. 
for me,  I work at a music store repairing amps.   and every week we have at least one amp come in where the owner says "it doesn't sound like it should"  (usually meaning the amp doesn't sound like other amps of the same make and model)  9 times out of 10 its because somebody decided to mod the amp.  usually its poorly done.

So my position is coming from that perspective.  to me is a shame to see excellent vintage amps turned into something they aren't. 
Especially when as I said there are plenty of cheap vintage amps that would do the job better... And there are dozens of companies making kits that can be made.  There are so many kits available and they are perfect candidates for tinkering.  because they are kits.  they're not a piece of history,  they're not an amp with vintage value.

So, from my perspective it just doesn't make sense.


as for cathode bias,  most of my amps are cathode bias.  I really don't own that many loud amps.  I agree with Rob in general that they have an excellent sound.  I generally prefer it to fixed bias.

however, any issue with the bias supply in the V4 can be taken care of.  I use mine regularly, and my coworker uses his V2 even more often.  never had issues with the bias.  we've seen dozens of V2s and v4s this year and none of them have come back with bias issues.

That being said, I think that the Cathode bias mod is easy to do and undo without permanently modifying the amp. so as long as people understand that it does make the amp quieter, and if is done in a non-destructive way,  I really do not have a problem with it.
I've even done it myself to see how it sounded.

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#24 2010-12-22 08:19:56

gsb
Member
Registered: 2010-02-10
Posts: 58

Re: 6550 Drop-In Compatible Tubes?

hangman, and what do you think of the idea of changing V4B's fixed bias to bias pot and ALSO balance pot between tubes? My tech says that bias voltage is OK for that tubes I have (6L6GC) but they dont work ideal in pairs, I mean, one pair is beaten more than the other. This causes also low hum after going from standby to playback mode (like low G note). He suggested installing small balance pot. hm?

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#25 2010-12-22 16:26:57

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 6550 Drop-In Compatible Tubes?

that is another mod that is easy to do in a non-destructive way.
go for it.

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