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#1 2012-09-30 17:41:41

pellek
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Registered: 2012-01-30
Posts: 7

Homemade V4

Hi, i recently build my own V4 from scratch.

I got it fired up, and it sounds amazing!!!

But when i cranked it up the first time, with a RAT in front of it, it sounded amazing, but after some minutes, one of the power tubes, was riding the lightning. Blue sparks went inside the tube. After that the fuse blew. Installed a new fuse, and try'd again. The tube seems to be shorted.

What can cause this problem?

The Bias is -52V on all grids, the signal voltage is 230Vpp.

I only tried it with 2 PT on a single speaker impedance according to the 1/2 output.

Greetz,

Pellek

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#2 2012-10-02 17:57:15

pellek
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Registered: 2012-01-30
Posts: 7

Re: Homemade V4

Ok, seems to be a 470Ohm resistor that got open.

I installed some 2W 470Ohm resistors, gonna replace them with 1k 5W resistors. To reduce the screen voltage a little. Plate and screen voltages seems to be very close to each other.

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#3 2012-10-02 19:22:25

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

Re: Homemade V4

The 1k resistor won't reduce the screen voltage by any appreciable amount.  What it will do is help keep parasitic oscillations in the power tubes to a minimum.   So I do recommend the 1k resistors,  but just wanted to clarify the reason.

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#4 2012-10-03 15:54:38

pellek
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Registered: 2012-01-30
Posts: 7

Re: Homemade V4

Ah ok, thanks for the info. I'm just a beginner in amp building. (First project) 

Btw, the reverb isn't working yet. Does any one know the voltage of the signal that goes to the reverb can, and to the PI?
I just hear a little reverberation going on.

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#5 2012-11-11 11:57:05

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

Re: Homemade V4

I'm interested in doing a scratch build (with some variations...may actually happen.

Did you use stock replacement transformers?  How did you cut the holes, if so?   I was thinking of these chassis blanks (http://www.turretboards.com/guitar_ampl … _blank.htm)
...the V-series transformers are lay down, so that means doing a cut out, which probably means a jigsaw.  In an ideal world I could do everything with a drill...curious to the approach you took.

Last edited by Liquids (2012-11-11 11:57:42)


Matthew

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#6 2012-12-16 11:58:09

pellek
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Registered: 2012-01-30
Posts: 7

Re: Homemade V4

I'm busy building version 3 right now. Version 1 and 2 were build on a marshall super lead chassis, wich is a little on the small side.

I drilled the holes with a special drill, i don't know it's name in english, but it is a multiple stage drill that goes from 4 to 32 mm in steps of 2mm.

In version 3, i drawed the complete chassis in autocad, and had it lasered out at my work. It's gonne get powder coated, and engraved for finish. I couldn't find a chassis that was big enough. This one is 630x250x80 mm in alluminium.

The transfo's i use are stock ones for a V4. But, since version 2 worked, but wasn't maintenance friendly, and to damn loud, i've decided to go for 2 output tubes only, and only one channel, (since they are both identical) so the impedance will be like 4, 8 and 16 Ohm, wich is more practical than 2, 4 and 8 Ohm.

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#7 2012-12-16 16:29:55

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

Re: Homemade V4

Interesting.

The drill bit you're talking about is commonly refered to as a 'unibit,' though that is more of the popular brand "Irwin" calling it that.  It can also be referred to as a 'step' drill bit (and possibly other things).

I've got an 18"x8"x2" chassis in hand (450mm x 200mm x 50 mm); it is made of a thick and sturdy aluminum, which I got cheaper than the one I linked to before.  I am using it as a prototype chassis, as I haven't settled on anything for sure since I'm not 100% cloning.   Indeed, that chassis is too small for anything but a very tight and precise layout of the transformers, tubes, and caps anyhow, or someone who's used to doing this.   It was the biggest size available from this particular producer, which I took anyhow, since it was affordable...

I've mounted the chassis to a large wooden board anyhow, and I have about 16cm of wood on either side of the chassis to work with for prototyping.  I think I will mount the transformers to the wood board, rather than the actual chassis, in order to have sufficient space for everything to fit and connect up more quickly, comfortably, and such, I suspect, though the power and preamp tube sockets and such will all be on this chassis...or maybe even the power transformers will be mounted, but not the output transformer.   Dunno.   It's a prototype for figuring out what the final build will be.

I'd like more space than I have on the chassis for the final build, even though I am not going to have much for 'controls' on the amp, and it will be more or less just a power amp, taking it's basis from the EXT in of the V-series circuit(s) to the speaker output.   I may experiment with a negative feedback control pot, or something like the 'cut' control that VOX amps use in the phase inverter, as I prototype, but other than that, I don't really need any control.  I suppose I could put a 'master' volume control on it instead of, or in addition to any preamps I build and drive it with having their own volume control... 

The transformers I got are not clones of Ampeg (I have an original pair of V4b transformers anyhow).  The transformers are 'stand up' since I'd rather not be bothered with the work or commitment to such a large chassis cutout and this prototype chassis/board may serve other future work.  So I may actually have more than enough room to work with *inside* the chassis for circuitry; multiple individual boards may be the way to go for the minimal components I'll need for the circuit.   

I have 2 separate power transformers to use though, for now, so I that can have a lower voltage on the screens (like the later V-series distortion models had and 6550s/KT88s seems to prefer and be designed for, or, last longer when configured that way).

I am only going to run 2 x KT88s; I don't need 4 tubes, no matter what, here; the benefits of 4 tubes over 2 tubes is minimal in volume, while the cost of transformers and 2 more power tubes is very significant.  2 Kt88s run in any configuration should be plenty since 2 6L6GCs run in a V4b already is.

The power output with either the separate screen tap for the 2x KT88s with high voltage on the plates, or run with an UL output configuration, or, even if I revert to a choke. Whichever configuration I pick, I should have be plenty of headroom, low end, and 'authoritative' tone with 2x KT88s and this big hi-fi power transformer rated for 100W 30hz-30k, since I'm using it for guitar and with guitar speakers.  It may even be enough for bass, depending on the situation so long as I'm not trying to compete with a big loud drummer, but I turn down most if not all gigs at this point, and I play guitar predominantly, bass minimally, and I'm not afraid of tube-less bass amplification if/when the need arises, etc....     

The blank chassis that I linked to before is more expensive than the one I have for prototyping and longer like the one you mention, though not quite as deep or thick as the one you have.  I'll have to see how the prototype chassis works out and/or order the largest and most sturdy chassis I can reasonably get and drill and afford.

For now, I am mostly experimenting with solid state/op amp/jfet/bjt based preamps with inductor/gyrators and other means of midrange control, running them into my V4b (sometimes the input with the midrange pot set 'midway' if I'm experimenting with active midrange variations on the preamp) or sometimes the basic preamp circuitry, plugged right into the EXT in of my 1970's Ampeg V4b.  The power amp work comes later, or when I've had sufficient fun with preamp work. 

Thus far, I hear little to no major detriment with the EXT being driven by solid state equivalents, as compared to plugging my guitar/pedalboard right into the amp's preamp. I have a number of pedals, but even clean with little in the path but buffering to 'drive' my rack-mount TC Electronics unit, which provides all my delay and reverb.

It's interesting to remember that, more or less, running right into the EXT in with a guitar (With or without buffering the signal) is loud! The one tube *gain* stage after the EXT in  - which is important and needs to be included in my 'power amp' design since my knowledge is limited and my desire to duplicate it is high - provides a place for negative feedback from the output transformer to flow, plus the gain from the phase inverter, even with it's low gain tube, the EXT in to speaker circuitry still provide enough gain to be load and even approach breakup with the average guitar

So I've found, for a preamp, and for a clean sound, the focus is upon the tone controls and a clean, 'transparent,' low-noise way to CUT gain even if after boosting gain first or recovering gain lost from the tone controls.  I have to get pretty high in the V4b's volume control's setting when plugged into the front input, so as to approach the volume of plugging right into the EXT in.   

Before working with the solid state preamp circuit options, I was thinking 'tube' and that I would need the gain staging found in the V4 from the preamp to drive the rest of the circuit sufficiently; effectively, for a clean sound, I predominantly need a way to CUT gain since the circuit from EXT in, and onward, already has gobs of amplification power for a low-level guitar signal to be amplified for loud and clean, even with 2 6L6GC tubes.   It's interesting to think about how much gain is thrown away in the range I typically set the amp's volume control, stock, clean.

Last edited by Liquids (2012-12-16 16:45:03)


Matthew

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#8 2012-12-16 18:12:07

pellek
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Registered: 2012-01-30
Posts: 7

Re: Homemade V4

That chassis is still on the small side, if you want those transfo's up there. But it might work if you only want to make the power amp section.

The biggest problem with this model, is the power supply. It just takes a lot of space. Even with those can type elco's trough the chassis.

The chassis i'm working with now, is very big, but even then i have to cheat a little to fit it all in, and make everything as accessible as possible.

I have to place the preamp board vertical, instead of horizontal, to save a lot of place. Like in the Sovtec MIG amps.

I even made the complete layout in autocad. It took me a while, but you can easily overview the result, before you even start building.

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#9 2012-12-17 02:31:22

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

Re: Homemade V4

did you make a PCB layout, or a turret/stripboard/etc kind of layout?
I don't have any CAD software, but if you can make your layout something that can be viewed without a special program, I'd be interested...especially the final triode gain stage & phase inverter - might eventually save me some time.


Matthew

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#10 2012-12-17 19:48:25

pellek
New member
Registered: 2012-01-30
Posts: 7

Re: Homemade V4

I work with eyelets, it's a little easyer to change something afterwards.

If you just need the final gain stage and the phase inverter, it should be easy to figure out. Just a small board will do the trick.

The power supply will be easyer too, since you don't need the reverb supply.

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