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#1 2012-02-11 23:47:37

AEB-1
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VT-22 Power Supply Ripple - hum

I just rebuilt a VT-22 rev-F. I replaced every cap and any resistors found out of spec. The previous owner had done a bias mod so I returned it to match the schematic. The amp had a shorted output tube and took out the hum balance pot so I replaced it as well and put in a new set of Sovtek 7027's. The amp plays really well and sounds great at volume but it has a mild 60 cycle hum and with nothing plugged in (no signal) and I open up the volume or reverb controls it amplifies the hum a lot. I used F&T main caps and CE 40-40-40. 
With a scope I have the following:
A = 541 DC    20v P-P Sawtooth
B = 536 DC     2v P-P Rounded
C = 424 DC     .2 P-P Rounded
D = 365 DC     ripple too small to measure
E = 331 DC     ripple too small to measure
Bias -62 DC    Clean - no ripple

When I look at the main F&T caps, there is 10vPP on each =20v P-P. Is anyone familiar with these symptoms?
Thanks,
Tim

Last edited by AEB-1 (2012-02-11 23:48:21)

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#2 2012-02-22 16:05:21

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

Re: VT-22 Power Supply Ripple - hum

I'm shooting from the hip here since I've not used my scope to look at the DC before...but I can't imagine that 20v P-P of ripple on the plates is right.  Or, does it smooth out noticeably by the time it crosses the OT and reaches the plates? 

Hopefully someone else will chime in, sorry I can't help more.  As always, re-check your work though especially around the filter caps.  Or power down and drain everything, use a capacitance meter to measure capacitance from point A to ground to be sure you're getting the filter capacitance you think you should?


Matthew

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#3 2012-02-22 17:31:28

AEB-1
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Re: VT-22 Power Supply Ripple - hum

The 20v ripple is through the OT on the plates. Ive rechecked my work doens of times. I subbed in a complete set of tubes. I figured there has to be too much load on the filters but the 7027s appear to be around 41 ma idle. I clipped another 100uf across each primary cap and the ripple only went to 10 volts. The standby diode tested good but I replaced it in case the ground path had resistance. The new diode made no difference. I measure 0.0 ohms to chassis ground from every ground in the amp. The only thing in the power supply (other than ripple) that is questionable is the "C" voltage is 424 and the schematic says 400. But that should be less load on the filters. The ripple gets better through the filter stages but it's enough that the preamps pick it up and amplify it when the volumes are increased from zero. I'm at a loss so any help that anyone has to offer is welcome.

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#4 2012-02-22 17:39:12

hangman
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Registered: 2006-09-04
Posts: 1848

Re: VT-22 Power Supply Ripple - hum

AEB-1 wrote:

When I look at the main F&T caps, there is 10vPP on each =20v P-P. Is anyone familiar with these symptoms?
Thanks,
Tim

Can you explain what you mean here?  Do you have 20vp-p. or 10v p-p?

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#5 2012-02-22 19:36:44

AEB-1
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Re: VT-22 Power Supply Ripple - hum

Total P-P = 20v   The P-P voltage is 10v across each of the F&T 100uf caps just the same as the DC is equal across each of the two caps.  I was trying to detrmine if one of the two new F&T caps was bad. I'm pretty sure they are both good.

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#6 2012-03-02 02:55:51

AEB-1
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Re: VT-22 Power Supply Ripple - hum

Update: Amp still not fixed but I managed to take some readings on another VT-22 that is dead quiet with the original filter caps and stock bias circuit. This amp plays great.

This clean amp has the following:
A = 546 DC    12v P-P Sawtooth
B = 544 DC     1v P-P Rounded
C = 422 DC     .1 P-P Rounded
D = 364 DC     ripple too small to measure
E = 332 DC     ripple too small to measure

The DC readings are nearly the same but the Peak-Peak readings are 1/2 of the noisy amp readings. The wiring of the clean amp looks the same as the noisy amp. What else causes the ripple to be affected when the DC voltages are essentially the same?

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#7 2012-03-02 05:28:01

hangman
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Re: VT-22 Power Supply Ripple - hum

current draw, ripple frequency.

is it possible that one of the bridge diodes is open?   

or is it possible that you replaced the bleeder resistors with values that are too low?

how much current does the bad v4 draw from the wall,  and how much does the good one draw?

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#8 2012-03-02 06:22:40

AEB-1
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Re: VT-22 Power Supply Ripple - hum

Good Ideas. I verified the bleeders are 100k - I reused the originals. I checked the bridge diodes in circuit and they checked ok. It's after midnight here in Tennessee so tomorrow night I'll pull the diodes and recheck them out of circuit. If the diodes are ok, I'll check the input current. Would I still see full DC voltage if a bridge diode is open?

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#9 2012-03-07 15:19:37

AEB-1
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Re: VT-22 Power Supply Ripple - hum

I pullled the diodes and checked individually and they are ok. I rechecked the ripple on the main filters and it it 120hz which indicated the fullwave bridge is working. So I decided to pursue hangman's suggestion that is has to be related to current. Reasoning that the output section is the only section that can draw significan current I pulled the output tubes and the ripple went to 6v p-p. I built 4 bias meters so I could see all four 7027s at the same time. Well it turns out my new matched quad Sovtek 7027 are not matched at all. I recorded the current and relative mu from my Hickok 6000a.  (Bias = -62v)
V8  26ma   5100 mu
V7  30ma   5500 mu
V6  40ma  >6000 mu
V5  34ma   6000 mu
I pulled the output tubes from the quite amp installed in the noizy amp. They are vintage GE and all read 25ma. The ripple is about 12v p-p. The amp is usable and it's just a little bit noisier than the quiet amp. I put the sovteks back in but paired the 40ma+26ma and 30ma+34ma so the load would be more balanced. The amp is more quiet than original but not as quiet as the GE 25ma tubes. I've ordered a matched set of JJ 7027 to try in it. I'm surpised that 25ma seems to be the ragged edge of keeping the ripple low. I'm fixing these amps as a hobby. What do you experienced guys see as the typical bias current range? I'll post the results of the JJ 7027 set when they arrive. Thanks for all the help.

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#10 2012-03-08 04:26:37

hangman
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Re: VT-22 Power Supply Ripple - hum

I don't think it's a matter of current as much as a matter of imbalance.   
I will say flat out....  The sovtek 7027s are lousy.   I have only had bad luck with their 7027s as well as their 7591s.  Honestly both are just relabeled sovtek 5881wxts.   Which I think is a crummy tube off the bat. 

But I have put kt88s in a v2 and biased much higher than 25ma with little or no hum.   And most v4 s I work on end u p with about 33ma per output tube pat idle no problem.


Remember that when tubes are balanced in push pull amps there should be very little dc current in the output transformer.  When they are imbalances their is current...plus. Because the power tubes amplify end up amplifying the ac hum on the b+ since the two sides of the transformer are not effectively canceling each other out.   

I thin you just had a set of lousy tubes

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#11 2012-03-08 19:08:32

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

Re: VT-22 Power Supply Ripple - hum

25ma is a good place (I'd say max) for an amp running current 6L6GC tubes if you want any kind of reliability and lifespan.  I think I have mine at 22ma; but I didn't dial it by number, I dialed it in by ear, then measured the adjustable resistance I had dialed in - it was a hair within 75k!  I put a fresh 75k in there and will probably not screw with the bias ever forward.  That's just my experience and opinion of course.


Matthew

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#12 2012-03-09 22:33:38

AEB-1
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Re: VT-22 Power Supply Ripple - hum

Assuming my new set is balanced at 25ma, I'll end up with extra tubes from the bad set. Two are near 25ma and two are 34-40ma. What should I do with the two  that were 34-40ma with -62v bias? Should I look fo two more high gain tubes and increase the negative bias?

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#13 2012-03-29 16:52:54

AEB-1
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Re: VT-22 Power Supply Ripple - hum

final update:  Put in a new set of JJ 7027's. The current of two of them danced around for a couple of minutes but they finally all settled in at 25ma. The hum was much reduced. I was then able to find a couple of quiet 12ax7's. The amp is killer now. Loud as heck. Thanks for all the help. Tim T (AEB-1)

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#14 2012-03-29 19:09:09

hangman
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Re: VT-22 Power Supply Ripple - hum

Tim, glad you figured it out!  the v4 is pretty darn quiet when functioning right.   

use ear protection!  ha ha

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