BF Bassman low output (totally stumped)

[nateflanigan]

Hey all,


A friend dropped off two BF bassman amps a good one and a bad one and said, "please make the bad one sound like the good one".  The good one is an amp that I had personally serviced and done some mods on relatively recently.  The bad one had also been serviced but not by me, the tone was fine but the output is low.  I copied the mods from good amp to bad amp and installed a fresh set of tubes.  Better tone but still low output.  Voltages are very close between the two amps at all points and both amps are biased the same.


Tonight I hooked up a tone generator and scoped both amps.  With the controls set the same the signal is very close in both amps up until the plates of the power tubes.  I didn't count out the blocks on scope but the difference in signal was considerable and obvious.  I went over RG Keen's basic OT resistance test, everything looks normal.


Any ideas?  The only thing I haven't done is disconnect the OT and run voltage through it.  Is it possible to disconnect the OT primaries and scope the signal at the plates of the powertubes or will they freakout without a load?


Thanks guys!

[tubeswell]

... Tonight I hooked up a tone generator and scoped both amps.  With the controls set the same the signal is very close in both amps up until the plates of the power tubes.  I didn't count out the blocks on scope but the difference in signal was considerable and obvious.  ...

Any ideas?  ...

Have you tried a set of new output tubes?

[labb]

Might want to test the OT like this. Can show a fault the resistance test will not.    https://www.premierguitar.com/articles/the-super-secret-transformer-tester-1

[shooter]

installed a fresh set of tubes.

before you get to deep, did you swap speakers, and verify the spkr jack if it has one

[nateflanigan]

Thanks for the replies...


Just to be crystal clear...


- Yes, fresh set of powertubes. 


- These amps are 60's era Bassman heads.  I've tested them both with the same cab, as far as running a tone from my function generator through the amps and scoping and comparing various points, that's done using a dummy load.


- As far as the speaker jack is concerned, The signal (as viewed on my scope) is the same from amp to amp up until the plates of the power tubes.  So the issue is one OT away from that point.


Labb, I agree that further OT testing is the next step.  I've seen that article but never performed that test, hopefully I can track down an NE-2 bulb locally, no more radioshacks around here.


Keep the ideas coming!

[labb]

If all else fails swap  the OT's. Had a similar problem with a silver face bassman. It would play but low volume and distorted. Was a faulted OT.

[labb]

Take a read through this thread to see some of the discussions of the situation I found.  http://music-electronics-forum.com/t42418/

[nateflanigan]

Thanks, there's some good intel in there.  I think I have a OT around I can clip in, maybe, might be for a deluxe though.


Regarding the neon bulb test, it's not exactly clear to me, do we need to disconnect all the OT leads including the center tap so nothing is loading anything?


I've used this method to measure the impedance of an unknown transformer... http://www.marstran.com/Measuring%20Primary%20Impedance.htm
It's quick and easy, might reveal something.

[sluckey]

Substitution is a valid troubleshooting technique. It's quick and will tell you exactly what you want to know. Even a Deluxe OT will work for a quick test.

[nateflanigan]

Last night I got a little more work done.  First I performed this voltage test on the OT...


http://www.marstran.com/Measuring%20Primary%20Impedance.htm


The results seemed to indicated that the transformer is fine, I fed in 1v and measured about 0.031 volts on the secondaries which calculates to about 4k primaries.  None the less I disconnected the OT and tacked in an OT for a deluxe reverb I had lying around, the output as viewed on the scope is still way too low.  Meaning the signal going into the power tubes is stronger than the signal exiting them.  I also experimented with disconnecting the NFB on both amps, it had the effect you would expect but relative to both amps, meaning on the bad amp the signal is stronger without the NFB but still about half the level of the signal coming out of the good amp without the NFB.


I had had a long day at work and at this point was pretty hot and frustrated so I'm worried that I wasn't in the best state of mind and may have made mistakes. 


I'm running out of ideas, .  The only thing I can think to do at the moment is swap the OT's between the two amps.


Any other ideas would be greatly appreciated.  Thanks!

[shooter]

Voltages are very close

does that include the screen V @ G2?
have you ohmed out the screen R's?
have you re-tensioned/scrubbed the sockets?
do you have 1ohm bias R's installed?  If not maybe tack in some so you can monitor current "real-time"

[nateflanigan]

does that include the screen V @ G2?have you ohmed out the screen R's?have you re-tensioned/scrubbed the sockets?do you have 1ohm bias R's installed?  If not maybe tack in some so you can monitor current "real-time"

Yes, screen voltages (at G2) are 415v (while running signal through the amp) on the bad amp, I forget the exact number on the good amp, it was slightly higher maybe 420v.


I have measured the screen resistors, I think they ohmed out at 486r if I remember correctly.  Sorry to sound foggy but I'm at work now and not in front of the amp.


Sockets, that I haven't done.  They are actually quite stiff on the bad amp.  I was beginning to wonder about them but felt like I was grasping at straws.  plus is it likely both sockets are damaged in the same way?


I haven't done the 1 ohm bias resistors.  That's a very interesting idea, I think I can pick some up today and test tonight.


Thank you.  It's always very helpful to talk through these issues.

[shooter]

felt like I was grasping at straws

we're already in the weeds, n, well, up here the farmers use it for straw, I think you're on track

time n patience will nail this one, hang in there

[tubeswell]

Last night I got a little more work done.  First I performed this voltage test on the OT...


http://www.marstran.com/Measuring%20Primary%20Impedance.htm


The results seemed to indicated that the transformer is fine, I fed in 1v and measured about 0.031 volts on the secondaries which calculates to about 4k primaries.  ...

Doing a VAC test of an OT with no secondary load can be deceptive. A partially shorted unloaded OT can give a VAC test result that appears fine, but when you hook up a load, the short can appear.


Another way of doing a VAC test is using a VAC source where the source output is reduced by something like a 5:1 or 10:1 voltage divider. If the output (which you connect to the OT) measures a lot lower than expected (or even OVAC), there will be a short in the OT.

[shooter]

I disconnected the OT and tacked in an OT for a deluxe reverb I had lying around, the output as viewed on the scope is still way too low.

a VAC test of an OT with no secondary load can be deceptive.

you are correct, I got sideways by this, just idling everything was fine, hammering it reveled a partial short on 1/2 the primary side

for  fun, did you scope the secondary's of both amps?  I also had a bad probe send me rabbit hunting for ghosts because I only scoped the HV side

[nateflanigan]

Innnnnnnnnteresting.  When I did the VAC test I didn't "look" at the waveform I just measured the voltage across the secondaries with my meter. 


I'm about to head home and get back into it.  I'm not sure if it's less work to just swap the OTs between the two amps or conduct further tests.  I think to start I'll pull the power tubes from both amps, hook up the dummy load to the good amp, feed it my test signal, measure and scope the secondaries.  Then do the same for the bad amp and compare results.


Fingers crossed...

[nateflanigan]

Well...


Re-tested the OT as described above.  Both transformers perform the same with and without the dummy load, waveform looks the same on the scope, same voltage readings on a meter.  I got pretty aggressive with the test tone, I started at 1v and cranked it all the way up to 7v, signals were nice and clean either way.  Really seeming like it's not the OT.


I wasn't able to grab any 1 ohm resistors but should be able to tomorrow.  I like the idea of monitoring how the tubes are conducting current.

[tubeswell]

Tonight I hooked up a tone generator and scoped both amps.  With the controls set the same the signal is very close in both amps up until the plates of the power tubes. 

This could also be caused by the PI output. Remember that the LTP is a differential amplifier which amplifies the difference in the signals 'applied' to each grid. In a LTP, the non-inverting side's grid is supposed to be decoupled from any signal, so that all of the amplification comes from the grid signal on the inverting side. But if there is DC voltage leaking through the decoupling cap, or if the board is leaky, that could cause the differential amplifier to 'function' differently.

[nateflanigan]

Hmmm, I don't 100% follow that, but that's ok.  It's easy enough to check.  FWIW both the coupling cap going from the summing resistors to PI and the 0.1uf cap on the other side of the PI have been replaced (not by me).


If I put my scope on the plates of the PI I can see a nice clean wave form (one side out of phase from the other) equal in amplitude.  Same results on both amps.


I should clarify and say that I understand the PI as a differential amplifier, I don't understand how a healthy looking signal coming out of the PI would pollute the signal down stream, I'm probably thinking about this wrong.


What does it signal that the waveform looses amplitude at the plates of both power tubes? 

[shooter]

I like the idea of monitoring how the tubes are conducting current.

ya, me too.

I started at 1v and cranked it all the way up to 7v,

was that were the guitar plugs in, or just injecting into the OT?

do me a favor, inject say .3vac where the guitar plugs in on the good amp, and look at speaker with scope, repeat with bad amp

[tubeswell]

If I put my scope on the plates of the PI I can see a nice clean wave form (one side out of phase from the other) equal in amplitude.  Same results on both amps.


I should clarify and say that I understand the PI as a differential amplifier, I don't understand how a healthy looking signal coming out of the PI would pollute the signal down stream, I'm probably thinking about this wrong.


What does it signal that the waveform looses amplitude at the plates of both power tubes?

If the signal at each PI plate is roughly the same and in opposing phase, that tells you the differential amplifier is sending out opposing signals. If the signal is the same sort of clean sine wave that you are feeding into the inverting stages grid, then the signal probably isn't being contaminated by anything untoward at the other (non-inverting side) grid. If the wave amplitude at each plate is what you expect, given the gain of the LTP, then it probably isn't a PI problem.

The only other thing I can think of, is one of the coupling caps to the output tubes is leaking DC

[nateflanigan]

was that were the guitar plugs in, or just injecting into the OT?do me a favor, inject say .3vac where the guitar plugs in on the good amp, and look at speaker with scope, repeat with bad amp

It's funny, when I write out a post I feel like I'm being as clear and detailed as possible but inevitably I fail to communicate the whole picture, I'm sure this happens to everyone.  Thanks so much for sticking with me on this.


I have done both.  I.E. I have fed ~0.3vac to the input jack (where the guitar plugs in) on both amps and scoped the signal at every available point in the audio path of each amp.  The signals are close enough to call the same at each point (after the 220k summing resistors, after the driver tube, before the PI, each output of the PI both before and after the coupling caps to the power tubes, at the grids of the power tubes).  It's only when I scope the plates of the power tubes that I see a different signal on the bad amp.  The signal coming out of the power tubes is weaker than the signal going in, and the problem is the same on both tubes.


I have also pulled out the power tubes and connected the tone generator directly to the primaries of the OT on both amps and observed the signal at the speaker jack both with and without a dummy load.  In either situation the signal is identical on the good and bad amp. 

The only other thing I can think of, is one of the coupling caps to the output tubes is leaking DC

I think this is what's driving me crazy, there's really not that much going on at this stage in the amp.  I'll check all the caps around the PI for leakage.

[shooter]

I feel like I'm being as clear and detailed as possible

I do the same,
get some 1ohmers, even 10ohmers will work for testing
the answer is just around the corner, somewhere

[nateflanigan]

It's the OT.


After installing the 1 ohms and monitoring the same current at all power tubes (while amplifying my test tone) and checking caps for DC leakage I said f'it and swapped the OT's.  The problem followed the bad OT, the bad amp is now the good amp and vice versa.


Let this be a lesson, transformers are WILEY!!!

[shooter]

It's the OT.

now your toolbox has new an shiny knowledge
I spent about the same time on an OT in a GA-5t
and, if time allows, it's always funner to sneak up on a gremlin

[Sonny ReVerb]

Nice work. Perseverance pays off!

[labb]

Take a read through this thread to see some of the discussions of the situation I found.  http://music-electronics-forum.com/t42418/

Well, Well, Well....Now I don't feel so bad that I went through the same thing. Good work.

[nateflanigan]

Thanks guys, it's so meaningful to have sounding board for this stuff.

[John]

I was following this with great interest (had nothing to contribute) It's funny: it's hardly ever the transformer, but this time it was. Congrats on chasing it down!

[tubeswell]

It's the OT.


After installing the 1 ohms and monitoring the same current at all power tubes (while amplifying my test tone) and checking caps for DC leakage I said f'it and swapped the OT's.  The problem followed the bad OT, the bad amp is now the good amp and vice versa.


Let this be a lesson, transformers are WILEY!!!

That was my initial thought. (I thought you said you swapped the OT a couple of days ago?)