Marshall Amp Forum - View Single Post - Preamp Tubes?

MartyStrat54 · 04-26-2009, 03:38 PM

I'm a wee bit bummed... My beloved 12BZ7 that I got from you Marty... Is going bad... I replaced my power tubes*, re-biased, and discovered exactly where that rattling sound I have lately been hearing through my speakers was coming from... The 12BZ7. Color me sad.

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I was over at your other place for a while, but thought that I would come over and talk about the BZ7. To me, it seems as if maybe the amp just doesn't like them installed permanently. Some circuits place different voltages on the BZ7 over what an AX7 would see. Since it has less plate resistance, maybe it is getting hotter than it should? However, in all of my old reference materials, a BZ7 is listed as a replacement/substitute for an AX7. Who am I to argue with all of these old books? The AX7 is a lot older tube than a BZ7. I don't think the BZ's were around until the early 60's.

One thing on EBAY that you see a lot. "SOLD AS IS/NO RETURNS." (This gives you a real secure feeling when placing that bid.) I guess I could send you another and if it goes bad, then it goes back to what I said a long time ago with BZ7 #1, "the amp doesn't like them." And I'd hate to see good tubes go bad if it is indeed the amp that is eating them. I'll let you make the call on this one.

Well ole Solar Panel (he-he) is really cooking with fire.

The guy must be a zillionaire going out and getting himself a new lunchbox. I wish I had a little lunchbox like that. I'd stick it full of groovy tubes (no, not those junkie Groove Tubes). I'd make it learn to wail and scream.

I can't wait to hear about it. Little amps like that are perfect to A/B tubes. I mean you have two pre's and two power tubes. Take V1 and change it out, you are going to hear things right away. V2 is the tone stack and a nice tube in there is going to make a big difference. Some nice USA NOS power tubes..."WHAM!"...you can hear the tone of the tubes. Big amps have all sorts of circuits that can disguise the tone of the tubes, but little amps, well they are sort of like a tube tester. They can sort out the good tubes from the bad.

I would love to have a NT just so I could put the ICBM and the BP in the tone stack. Super charge it with some hot American made EL84's and I'd be off to the races. (I still would have to build a 4 by 6 custom speaker cab. That would be "tits!" (As they say in California.)

Well, I'm looking forward to hearing from Joe and I hope it is all good. I don't like bad news, just fun, happy stuff.

04-26-2009, 03:38 PM	#167 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Preamp Tubes? - Who, What, When, Where, Why, and How? I'm a wee bit bummed... My beloved 12BZ7 that I got from you Marty... Is going bad... I replaced my power tubes, re-biased, and discovered exactly where that rattling sound I have lately been hearing through my speakers was coming from... The 12BZ7. Color me sad. *********************************************** ***** ********************************************** ******* I was over at your other place for a while, but thought that I would come over and talk about the BZ7. To me, it seems as if maybe the amp just doesn't like them installed permanently. Some circuits place different voltages on the BZ7 over what an AX7 would see. Since it has less plate resistance, maybe it is getting hotter than it should? However, in all of my old reference materials, a BZ7 is listed as a replacement/substitute for an AX7. Who am I to argue with all of these old books? The AX7 is a lot older tube than a BZ7. I don't think the BZ's were around until the early 60's. One thing on EBAY that you see a lot. "SOLD AS IS/NO RETURNS." (This gives you a real secure feeling when placing that bid.) I guess I could send you another and if it goes bad, then it goes back to what I said a long time ago with BZ7 #1, "the amp doesn't like them." And I'd hate to see good tubes go bad if it is indeed the amp that is eating them. I'll let you make the call on this one. Well ole Solar Panel (he-he) is really cooking with fire.The guy must be a zillionaire going out and getting himself a new lunchbox. I wish I had a little lunchbox like that. I'd stick it full of groovy tubes (no, not those junkie Groove Tubes). I'd make it learn to wail and scream. I can't wait to hear about it. Little amps like that are perfect to A/B tubes. I mean you have two pre's and two power tubes. Take V1 and change it out, you are going to hear things right away. V2 is the tone stack and a nice tube in there is going to make a big difference. Some nice USA NOS power tubes..."WHAM!"...you can hear the tone of the tubes. Big amps have all sorts of circuits that can disguise the tone of the tubes, but little amps, well they are sort of like a tube tester. They can sort out the good tubes from the bad. I would love to have a NT just so I could put the ICBM and the BP in the tone stack. Super charge it with some hot American made EL84's and I'd be off to the races. (I still would have to build a 4 by 6 custom speaker cab. That would be "tits!" (As they say in California.) Well, I'm looking forward to hearing from Joe and I hope it is all good. I don't like bad news, just fun, happy stuff.