Replacing preamp tubes?

[toejam]

I've got a Mesa/Boogie F-30 combo with two EL84s and four 12AX7s. It seems that even playing at low volumes now I get squealing feedback very quickly, and at loud volumes it's brutal! [img]/images/graemlins/eek.gif[/img] I guess one (or more?) or my preamp tubes has gone microphonic. Should I replace only one or two of the preamp tubes, should I do all four? Also, after replacing them, will I need to rebias the amp, or is that only when replacing the power tubes? This is my first tube amp and the first time tubes are needing replacement. Thanks! [img]/images/graemlins/smile.gif[/img]

[GodWentPUNK]

Re: Replacing preamp tubes?

You only need to rebias if your replacing your power tubes. I have the same problem with my MB and I think I only have to replace the 1 tube that is closest to the powertube (balanced ECC83S ). I'm going to order JJ high-gain preamp tube.

joe...

[Grandturk]

Re: Replacing preamp tubes?

Try this for a quick fix - pull the first preamp tube and the last preamp tube and swap them. See if that helps with the squealing.

The first preamp tube has the biggest effect on the sound, and if it is microphonic, the amp will squeal like a pig. A microphonic tube can safely be used as the second, third or fourth preamp tube and will cause a minimal, if any, amount of noise.

[toejam]

Re: Replacing preamp tubes?

Cool. Thanks, guys! I'll try the swap a little later and see what happens.

[CZSER]

Re: Replacing preamp tubes?

Grandturk, AGREED. I'll suggest to try different brands of 12ax7 also(only at v1 & 2), even a 12at7 for less gain(if you like it). This is a cheap way to find out wich brand suits your amp/tastes better(allways with the same balanced phase inverter: the one close to the power ones)

[CZSER]

Re: Replacing preamp tubes?

BTW, JOE: put a thread about high gain balacanced phase inverter when you try it out. I'm interested in your conclusions.

[Tooling]

Re: Replacing preamp tubes?

How dramatic of an effect does the PI tube have on overall tone? How does it fit in the equation?

[toejam]

Re: Replacing preamp tubes?

Well, I swapped the tubes around, still getting the feedback. I think I'll just buy some new tubes... most likely JJs.

[GodWentPUNK]

Re: Replacing preamp tubes?

I replaced both my power and preamp tubes all at the same time so I can't honestly say how much tonal difference the preamp tubes made. I can say that my JJ tubes do sound much better that my stock Marshall tubes. As for the PI tube, I wouldn't expect any difference other that what you'd were expecting.

joe...

[toejam]

Re: Replacing preamp tubes?

I just ordered 4 new preamp tubes... I went with Electro-Harmonix (read some great things about them). When I need new power tubes, I'll most likely go with the JJs.

[Shawn Lutz]

Re: Replacing preamp tubes?

Cool. You won't need to re-bias for pre-amp tubes. Also most boogies won't need rebiasing after swapping power tubes either, so they claim [img]/images/graemlins/smile.gif[/img]

[me wise majic]

Re: Replacing preamp tubes?

Anything more specific on the high gain JJ's like a number or something. I'm playing with my preamp tubes also...

[CZSER]

Re: Replacing preamp tubes?

[ QUOTE ]
Anything more specific on the high gain JJ's like a number or something. I'm playing with my preamp tubes also...

[/ QUOTE ]
Think they are measured & selected by retailer/distributor (Bob @ Eurotubes does the job for few bucks)

[Ozz666]

Re: Replacing preamp tubes?

Deinitely if you only wanna deal with the preamp tubes, replace them all at once. No biasing necessary. Pop them out carefully pop the new ones in.

[me wise majic]

Re: Replacing preamp tubes?

Thanks!