• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

NOOB ALERT! sooo omany questions.....

benindavis

Member
2013-04-07 10:04 pm
Hi everyone!

I recently came across a cache of old vacuum tubes. Most look used.. A few are in boxes. I have a feeling that most of them are from televisions and old home audio stuff. I have been able to identify a few of them. ( I have a '69 Bandmaster Reverb) But.. danged if I know if any of them are working .. I was wondering if perhaps there may be a member ( with a tester) close to the Sacramento area who might be willing to meet with me and give me a hand finding out if any of these things are working and worth anything for say a .. few choice tubes or a 12 pk or somethin?? I have pictures I can upload to photobucket if that would help

Thanks,
Ben ( Davis CA)
 
Putting a 12BH7A in place of a 12AT7 in fact would not be too serious, more by luck than anything else. In this particular case the pin outs match, same heater wiring etc. but just stuffing any tube in any tube socket just because it fits is "fraught".

The 12BH7A however would not run at the operating point expected by the Bandmaster. And would probably sound pretty awful.

Cheers,
Ian
 

benindavis

Member
2013-04-07 10:04 pm
WEEEEEELLLLLLLLLLL.. I did it.. turns out.. sounds freaking awesome! its a much darker tone.. I dont get the distortion at high volumes but other than that its pretty freaking sweet! It wasnt just any old tube really.. a dual triode instead of a twin triode. right?
 
Oddly [DF96] enough ... a musician friend had a similar experience, except with the 12BZ7 tube, again pinout-compatible with the venerable AX7. He asked "is this OK to do?", and taking a look at his amp, I confirmed it had a stout heater, and the usual resistor-capacitor bypass on the front end, so it was a go.

And the sound definitely got "darker", less distorted at the top (which for guitar players is a bug, not a feature, mostly), but in the middle region, was darn sweet. Very jazzy. So... some 5 years later, the 12BZ7 has been replaced ceremonially to show people, but it remains in the box for the gigs.

As to double-the-heater-current, we're not talking about serious current: the overall power draw (line tubes, phase inverter, push-pull 4-tube final) might increase 5%, maybe less. Clearly within design spec.

I highly recommend experimenting with these "front end tubes" in guitar amps. Just be sure that the whole amp is tube based, and not one of the dumb modern ones with a single 12ax7 (typically behind a cute orange LED backlit or underlit window!) in the front end, and all silicon after that. The double-amp heaters might well be way beyond that capacity of the design! Only ALL TUBE is safe.

12AU7, 12AT7, 12AY7, 12BH7, 12BZ7 ... all good, different gain! Its especially fun to try different manufacturers' substitutes - comparing the Ruskies to the Chinee, to the German's NOS (since the make them no more). The front-end tube of the guitar amplifier has one of the most significant roles in "sound shaping" of any of the tubes.

Have fun!

Also check out the "franks tube pages" sites. It is NOT easy to find all the substitutes, but with some scanning and a pad of paper, you will locate 50 or so different types to give you stuff to try.

GoatGuy
 
As to double-the-heater-current, we're not talking about serious current: the overall power draw (line tubes, phase inverter, push-pull 4-tube final) might increase 5%, maybe less. Clearly within design spec.
In case of a single heater supply, agreed, but I have seen designs with separate heater windings or PSU's for pre- and power tubes. In those cases, the extra 300mA might make components run too hot for comfort.
 

benindavis

Member
2013-04-07 10:04 pm
Thanks GoatGuy! yeah I tried a few different brands and 12AT7, 12AD7 amd 12AX7, They all sounded ok at lower volumes but in the mid level and high began doing some really gnarly distortion I do not prefer.. ( I play mostly blues and surf stuff) With the 12BH7A there is just enough growl to make it sound gritty, not.. er you know.. Black Sabbath Ironman if you will. and yes the mid is really full sweet and kinda jazzy!.. I will be posting a list of the tubes I have and if anyone cares to take a look and put in their 2 cents worth as to if any are worth investigation Id be most thankful!

Thanks to everyone for your input!

Ben
 
Sorry. I forgot that people keep posting guitar amp questions in the wrong forum. Yes, for a guitar amp anything with the same pinout is regarded as a possible substitute.

PS finding all valves with a particular pinout is easy - download the tool from Duncan's website. No, not PSUD2 - the other tool.
 
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benindavis

Member
2013-04-07 10:04 pm
Here is the Bandmasater Schematic FYI
 

Attachments

  • bandmaster_reverb_aa1069_layout.gif
    bandmaster_reverb_aa1069_layout.gif
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That is NOT a schematic, but a wiring diagram. Not that it is of no use, of course, but a schematic it be not. Lots of tubes in there! Since I see a bunch of other 12AT/AU types, they're fair game for substitution as well. Sometimes there's a big difference when middle-stages are substituted.

HOWEVER - please remember that the advice about "go forth and try it!" applies to one tube substitution at a time (except for the push-pull output, which I'm of the opinion really should never be substituted at all, willy-nilly. Sure, use different manufacturers' exact substitutes, but don't get into entirely different beam-pentode etc. configurations.

OK?

Good luck, and may the Force be with you.

GoatGuy