Blackface Bassman Toast???

[signalpaths]

hey guys, I just got a 66 black face bassman a couple weeks ago and I love it. However I was playin it yesterday and I heard some ringing like microphonic noise for a few seconds. Then there was a considerable volume drop followed by a burning smell and then nothing. At that time I switched it to standby and powered it down. I took the amp apart and gave it a once over and there was no apparent burn damage. The tubes looked fine too. I'm not sure about the filter caps because I didn't look at them. Does any one have any ideas what could have happened?? Did I fry my new beloved amp?? The bassman is a stock ab165 for the most part it is almost all original parts. I could only find 1 cap that wasn't stock, as I said before I have not looked at the filter caps. The one mod that has been done is the output transformer has been replaced and the output tubes converted to el 34s. I had read on the vintage amps forum that the black face bassmens were 4 ohm output. So I plugged it in to my 4-12 at 4 ohms and got fireworks inside the output tubes?? It worked fine for a few days after that but I was playing it safe running it at 16 ohms. Is it possible that because of the tranny change that my amp is operating at a higher ohmage and I fried my EL 34's even though they still appear fine?? Is there a way i can test the tubes??


please somebody help me
ERic

[Enzo]

Fireworks inside tubes is a pretty sure sign they are shot. Pull the power tubes and see if the amp will power up without blowing fuses. Then verify that B+ voltage is getting to pins 3 and 4 of all the power tube sockets.

If something smokes, it is a lot easier to see where it is coming from when the chassis is on your bench.

[Bruce / Mission Amps]

NEVER run a 4 ohm output tranny on 16 ohms... especially one with EL34s!!!
Open this thing up and see if the screen resistors are fried, blown apart or burned looking now or the sockets have arc/scorching marks.

[PRNDL]

The one mod that has been done is the output transformer has been replaced and the output tubes converted to el 34s.

Sacrilege!

[Regis]

NEVER run a 4 ohm output tranny on 16 ohms... especially one with EL34s!!!
Open this thing up and see if the screen resistors are fried, blown apart or burned looking now or the sockets have arc/scorching marks.

I was gonna say this but he said the OT had been replaced, I wonder with what and at what impedance?


I ran my SF Bassmans with EL34's for a couple of years and the difference was not that great. I eventually put them back stock and continue to run 6L6's.

[signalpaths]

I know its sacrilege but I got the amp with the el 34s installed already. But seriously thanks for all the good advice. I have actually thought of putting it back to stock output tubes. There is a reason I like fender amps, the clean sounds remind me of angels singing (6l6's). That being said I wouldn't mind having a channel with more gain but not at the expense of those golden clean tones. The screen resistors are fine someone before me changed them to 1 or 2 watt (i'm not sure exactly I'd have to look closer at them) metal oxide. In fact I see no evidence of smoke anywhere which is the weird thing since I smelled it. Sockets are fine far as I can tell. Tonight I will put it on the bench and pull the power tubes and follow Enzo's advice with the B+ voltage. Does anyone know what the B+ should be between on a 66 bassman(Enzo)?? How can I test the Z on the transformer?? I've heard there's a way on speakers by testing the resistance and then multiplying by a factor (reactance I presume). If this method works for a tranny too, how do I test for or find out what this factor is. What tranny wires do I test for resistance? Or am I just totally wrong about all this, I am quite new to amplifiers. lastly could someone (Bruce maybe?? I sure your busy so no rush) explain to me or post a link on why it is bad to run a 16 ohm cab off 4 ohm amp. I was under the impression that the only negative aspect of going up in ohms was a drop in volume and maybe some signal loss (highs).

Thanks for all the help

Eric T

[Regis]

Those are all good questions. The B+ in my Bassman heads averages around 450 volts. That can vary. The schematic sez different, it's a little lower there.

Here is how to determine unknown OT impedance.

Here is a blurb about mismatched speaker impedance.

Here is a place that has your amps schematic.

[signalpaths]

Regis Thank for the links and kind words you are a stand up guy. I really dig your site it is a wealth of info. I especially like the gear section I am a vintage gear junkie an love to drool over other peoples stuff. Looks as though you have made quite a collection. I am really interested in traynor amps, I just keep hearing great things about them. I have been told more than once that "a traynor is the best sounding amp I've ever played." But I don't want to hijack my own thread so maybe I will start a traynor thread.

[hasserl]

I don't like that statement on impedance mismatch. If you mismatch the impedance 100% lower that equals a direct short, no impedance at all. I think you mean half the rated impedance to twice the rated impedance is a safe range.

To answer the question of why it is bad to run 16 ohm load into a 4 ohm amp, the higher load will cause high flyback voltages that will cause voltage spikes on the output transformer and power tubes high enough to pierce thru the insulation of the windings and/or cause arcing at the tubes or sockets.

It is an often repeated ill advised internet rumor that it is safe to run a higher load on a tube amp than what the amp is rated for. That is just plain wrong. That advice works for a solid state amp that does not use an impedance matching output transformer, not for a tube amp that does. Tell the guy that gave you this bad info that he may owe you an output transformer.

[Regis]

Regis Thank for the links and kind words you are a stand up guy. I really dig your site it is a wealth of info. I especially like the gear section I am a vintage gear junkie an love to drool over other peoples stuff. Looks as though you have made quite a collection. I am really interested in traynor amps, I just keep hearing great things about them. I have been told more than once that "a traynor is the best sounding amp I've ever played." But I don't want to hijack my own thread so maybe I will start a traynor thread.

I'm an old michigan man myself, I grew up in Jackson. At some point when I get back up there I am going up to lansing to see Enzo.

Yes the Traynors are way cool. I played them for a couple of years but recently am giving them a rest and have been playing my Bassman heads through a new 2x12 I recently bought.

I have a little YGM3 traynor that is cool, been playing that a lot too. I just put an attenuator insert in it, check it out.