Blonde Bassman - SS rectifier sag?

[homebrewer99]

Amp gurus-

I have a 6G6-B Blonde Bassman that has been serviced appropriately (cap job, biased with NOS tubes, sockets/pots cleaned etc). Both before and after the servicing the amp has had the following issue:

The amp experiences extreme sag, even to the point of cutting out. This only occurs at higher volumes (above 5) and is even worse when the treble control is at maximum. A few caps that were leaking DC were replaced on the bass channel. This eliminated some scratchy pot noise, but the sag is still there. I'm confused because this amp has a SS rectifier. Could the diodes be failing? Perhaps a failing output transformer? Let me know what I should check.

cheers,
Homebrewer99

PS: the amp is running into two Celestion G12H Heritage speakers at a 4ohm load. Tubes are NOS 6L6WGB biased around 65% dissipation. Plate voltage is around 450V. No circuit mods have been made apart from replacing leaky caps and improving some grounding issues.

[ChickenLover]

Let me know what I should check.

First you have to verify that it is not external to the amp. All cables good? Speaker cab good? Make damn sure it's actually the amp.
Then I would go through all the tubes (replacing with known good ones) and make sure they are all working nominally.
Then I would go through the amp and measure/record all voltages (including your incoming wall AC voltage).

That will very often either solve or isolate the problem. If that doesn't get it...I then input a constant signal to the amp (I use 880Hz/100mV) and measure the AC voltages at every plate/grid.

After that I almost always know where or what the problem is.

[Blue Strat]

Amp gurus-

I have a 6G6-B Blonde Bassman that has been serviced appropriately (cap job, biased with NOS tubes, sockets/pots cleaned etc). Both before and after the servicing the amp has had the following issue:

The amp experiences extreme sag, even to the point of cutting out. This only occurs at higher volumes (above 5) and is even worse when the treble control is at maximum. A few caps that were leaking DC were replaced on the bass channel. This eliminated some scratchy pot noise, but the sag is still there. I'm confused because this amp has a SS rectifier. Could the diodes be failing? Perhaps a failing output transformer? Let me know what I should check.

cheers,
Homebrewer99

PS: the amp is running into two Celestion G12H Heritage speakers at a 4ohm load. Tubes are NOS 6L6WGB biased around 65% dissipation. Plate voltage is around 450V. No circuit mods have been made apart from replacing leaky caps and improving some grounding issues.

450 Vplate would be low for those amps with 120VAC in. What's your line voltage like? It almost sounds like someone has installed a series resistor for sag or maybe your power transformer has issues on the secondary, causing voltage drop and sag.

What type of power tubes are you using? Any difference in sag between the 2 channels?

[Jade]

I'm wondering if your "extreme sag" is not actually a parasitic oscillation occuring above audible frequency. The bit about it being worse with the treble turned up really points me in that direction. If it were simply power section sag, extra bass would probably amplify the problem, not extra treble.

The slightly low voltages could also be caused by this as well.

If nothing else pops up as bad, have a tech stick a scope on it, and look for a high frequency oscillation.

Jade

[Swarty]

I'm wondering if your "extreme sag" is not actually a parasitic oscillation occuring above audible frequency. The bit about it being worse with the treble turned up really points me in that direction. If it were simply power section sag, extra bass would probably amplify the problem, not extra treble.

The slightly low voltages could also be caused by this as well.

If nothing else pops up as bad, have a tech stick a scope on it, and look for a high frequency oscillation.

Jade

My thinking also. Look at the tubes when it chokes off, if they look like they are working this is what you are dealing with. I've had it bad on 3 of these. Shielding the first couple gain stages solved it on two of them. The 3rd was caused by too much time in a Michigan basement causing the board to soak up humidity and become conductant.

[Blue Strat]

I'm wondering if your "extreme sag" is not actually a parasitic oscillation occuring above audible frequency. The bit about it being worse with the treble turned up really points me in that direction. If it were simply power section sag, extra bass would probably amplify the problem, not extra treble.

The slightly low voltages could also be caused by this as well.

If nothing else pops up as bad, have a tech stick a scope on it, and look for a high frequency oscillation.

Jade

+1. That was a thought here too.

[dick wiewy]

Big "Me-2" over here. :BEER

My otherwise pristine 6G6-B exhibited these same exact conditions. I worked from the PI backwards never finding the culprit so to speak. I ended it's misery by shielding the grids on V1 & V2 and possibly moving some wires slightly during my probing. I still think there may be moisture wicked into the tagboard but it hasn't started causing me any headaches yet.
{I wish I had my O'scope back then!}

[homebrewer99]

Thanks for the info guys. The sag is worse on the Bass channel. I said that the Treble control increased the sag, but I should have also mentioned that placing the treble on max also increases a bit of gain. The sag seems to be related to volume/gain. I should also note that the amp is very loud and strong. It just cuts out if I dime it. I did some investigation and found some intertesting data...

The plate voltage is actually much lower than I thought. I measured 435V DC at the plates. The 6L6WGBs are hovering around 32/33 mA for about 14W each. This is about 61% dissipation. I will investigate the oscillation concept. As for a sag resistor, I'm sure there isnt one present in the circuit. David Allen revised the bias supply to make the bias adjustable via potentiometer. He also replaced the bias supply cap and a resistor.

Could lead dress be the cause of oscillation? I will contact David and see if he can scope the amp. I will also try it with different speakers/cables this weekend.

thanks again,
homebrewer

[RedMan]

It seems to me that oscillation would have to be really bad for the amp to actually cut out. Usually it just sounds bad and the volume gets reduced. I would put a meter on the power tube grids and see what the DC is doing when it happens.

[dick wiewy]

Redman, That oscillation can occur at inaudible frequencies and cause the power tubes to work extra hard at multiplying the output and to our ears there's simply nothing exiting the speaker to indicate this condition.

This is only one of the ways oscillations manifest themselves.

[RedMan]

Redman, That oscillation can occur at inaudible frequencies and cause the power tubes to work extra hard at multiplying the output and to our ears there's simply nothing exiting the speaker to indicate this condition.

This is only one of the ways oscillations manifest themselves.

I understand that point, mine was that it rarely manifests itself as the guitar signal being completly cutoff.

[tlpruitt]

How about adding grid resistors on the output tubes.

The 6G6B does not have the typical 1.5k "swamp" resistors on the grid of the output tubes like most Fender amps. I bet adding grid resistors will cure the cutoff problem.

-Tim

[homebrewer99]

Quick Update-

I messed with the amp this weekend and realized that the problem is only on the Bass channel. David Allen mentioned the possibility of a flaw in the phase inverter design. For now I will just play the Normal channel. Keep the ideas coming, though!

cheers,
homebrewer

[JJman]

How about adding grid resistors on the output tubes.

The 6G6B does not have the typical 1.5k "swamp" resistors on the grid of the output tubes like most Fender amps. I bet adding grid resistors will cure the cutoff problem.

-Tim


This sounds like the 1st step to me.