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#1 2009-01-16 20:06:16

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

more fooling around with my VT22

So I have the Day off today and decided to fool around with my amp a little. 
here is what I did:

I decided to try out a different screen supply configuration to drop the screen voltage to around 80% of the plate voltage.  which would bring the screen voltage back to a reasonable value, within the recommended voltage.  (450VMAX)   
so I used a voltage Divider circuit my friend Don uses in his amplifiers.   two 20Watt resistors, one 10K  one 2.2K.
The 2.2k goes off the B+to the screens and filter cap, and the 10k goes from that point to ground.   
I also decided to put a Choke in the amp  between power supply points C and D.   
wanted to put the choke between A and C but I knew the choke would not provide much voltage drop there... so I decided against it. 

I readjusted the bias and played on it for a while.   

it is quite a bit quieter.  and i attribute that mostly to the choke.   but the AC ripple on the screens seems to have dropped significantly also.  so perhaps that has something to do with it. 

the reverb did not experience this benefit,  so it does SEEM more noisy now.

otherwise the amp sounds really nice.   the bass seems more beefy.  overall I feel like it was a nice improvement.

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#2 2009-01-17 06:52:15

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

Re: more fooling around with my VT22

measured the power... after I readjusted the bias,  the amp still put out 95 watts clean.   
my thoughts are that the lowered screen voltage will cause less stress on the power tubes, allowing them to last much longer.

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#3 2009-01-19 12:04:32

rob
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Registered: 2008-10-28
Posts: 171

Re: more fooling around with my VT22

Hey sounds interesting. If I understand this correctly, it would appear that you are dropping the pre amp voltages as well because they are in series in this power supply?

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#4 2009-01-19 19:13:40

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

Re: more fooling around with my VT22

Rob,  no the screen supply (B) is Taken off of the first filter stage (A)  and the preamp voltages (C-E) are taken off of A as well. 
so dropping B did not effect the preamp voltages.  in fact,  because the Choke does not have as much of a voltage drop across it as the resistor it replaced,  I actually boosted the preamp voltages a little.  but this increase was rather marginal.

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#5 2009-01-19 20:34:24

rob
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Registered: 2008-10-28
Posts: 171

Re: more fooling around with my VT22

Hangman, I just looked at the schematic. I see what you mean. I spoke to a guy a few months ago who used 75V 5 watt zener diodes to drop the screen voltage. Screen elements  dissipate very little power. High voltage is only a problem if there is arcing going on. These amps output tubes appear to have a very long life because of the conservative bias Ampeg used. I find the tone warmer and smoother with better sustain when cathode biased. If you get a chance, give it a try.

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#6 2009-01-20 17:52:41

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

Re: more fooling around with my VT22

I've always had problems with people blowing tubes out.   I do realize that the screens dissipate little power,   but when the Data sheets say that the absolute max for Screen voltage is 450V  and ampeg is exceeding that by 80V,  dots start connecting for me.

I really dig your cathode bias mod.  but rather than mod my VT-22 I'm going to build one that is cathode biased.   I will still use this screen supply though.  the more I take note,  I notice that I really like the way amps that have a 60-80%screen to plate ratio sound.

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#7 2009-01-20 21:26:58

rob
Member
Registered: 2008-10-28
Posts: 171

Re: more fooling around with my VT22

I agree. What you have done is good practice. I was not aware that you have observed frequent problems with tube deaths. I have had no problems with any of the 6l6 tubes and can swap them without worry because of the cathode bias. I did experience some trouble before I swiched it. I noticed Ampeg also exceeded the max grid resistance in this circuit as well.

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#8 2009-01-21 17:39:15

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

Re: more fooling around with my VT22

you're right,  but they didn't do it as bad as most of the manufacturers of the day.   I have a friend who changes the grid resistor to 22k and the bias feed resistors to 82K. 
perhaps I'll give that a shot while I have mine open still.  I don't imagine I'll notice a dramatic change, but good practice is good practice.  the manufacturers set maximum ratings for a reason. 

-steve

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#9 2009-01-21 18:36:41

rob
Member
Registered: 2008-10-28
Posts: 171

Re: more fooling around with my VT22

I've changed them on mine. Absolutly not audible. I think that the tube spec sheets were very conservative in the ratings. We both know there are a lot of old V4s kicking around out there with the original  GE7027a.

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#10 2009-01-22 22:52:35

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

Re: more fooling around with my VT22

very true.  most of the power tube blow outs I see are Modern tubes.  so that probably says more about the tubes than it does the amplifier.

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#11 2009-01-29 06:21:57

cyanidegold
Member
Registered: 2008-11-01
Posts: 67

Re: more fooling around with my VT22

+1
I've run into some good luck involving jj's 6l6GC.   But not much else, even the ever lauded sed winged c's.

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#12 2009-01-29 16:42:47

rob
Member
Registered: 2008-10-28
Posts: 171

Re: more fooling around with my VT22

Hey Hangman, how does the amp sound with the dropped screen voltage? Is there a noticable change in tone? Where did you get a choke rated at 550V? I was thinking about trying the screen voltage lowering thing.

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#13 2009-01-29 17:04:05

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

Re: more fooling around with my VT22

I think there is a definite change in tone.  I felt the sound was more beefy, especially in the low end.  it just sounds more together.  you should give it a try.

The choke only has like 350 volts on it.  I pulled it from an old newcomb PA head.

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#14 2009-01-30 15:32:16

rob
Member
Registered: 2008-10-28
Posts: 171

Re: more fooling around with my VT22

Thanks Hangman. You mentioned over 95 watts output. How much power did it put out before the mod? I would expect a drop in output gain from the reduced screen voltage and less -V bias required.

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#15 2009-01-30 18:56:59

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

Re: more fooling around with my VT22

there was a measurable loss of power,  before It used to put out 100-105 watts.  I admit I expected a bigger loss,  but I'm not complaining.

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#16 2009-01-30 19:59:22

rob
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Registered: 2008-10-28
Posts: 171

Re: more fooling around with my VT22

Are you using a signal generator and a scope to measure output, or a dummy load?

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#17 2009-01-30 20:48:49

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

Re: more fooling around with my VT22

all three.

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#18 2012-10-20 19:04:25

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

Re: more fooling around with my VT22

hangman wrote:

I decided to try out a different screen supply configuration to drop the screen voltage to around 80% of the plate voltage.  which would bring the screen voltage back to a reasonable value, within the recommended voltage.  (450VMAX)   
so I used a voltage Divider circuit my friend Don uses in his amplifiers.   two 20Watt resistors, one 10K  one 2.2K.
The 2.2k goes off the B+to the screens and filter cap, and the 10k goes from that point to ground.

From what I've calculated, a 2.2k/10k divider puts 19.5W on the 10k, assuming 540V B+.

Question thought, more generally, but based on the same Idea.
If I have a handful of 10k/20W resistors:
If I put 2 in series, I have 20k resistance, but is that effectively capable of handing 40W, or still just 20W?

If I put 2x10k/20W resistors in parallel, I get a 5k resistor - is it capable of 40W?

If I make two such pairs of 2 PARALLELED 10k/20W resistors, and put them in SERIES, I have 10k of resistance, but at what power handling?


Matthew

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#19 2012-10-20 19:30:09

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

Re: more fooling around with my VT22

Because they are the same resistance you can just quadruple the watt rating.  As the current would be the same for all of them.  If the resistances were different,   That wouldn't be true,  as some of the resistors would have more current than the others, and the safe power dissipation would be dependent on the individual values and ratings,.


I took this mod out of my v4 not a week after I put it in.   It was an interesting experiment,  but it made the amp run even hotter than before,  and I didn't like that.  Plus I solidified my belief that the v4 is best left as the designers made it.    But it's a very interesting experiment.

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