Search found 758 matches

by jjman
Wed Dec 19, 2018 3:03 am
Forum: Fender Discussion
Topic: Blackface Princeton Reverb clone, el84 reverb driver mod
Replies: 17
Views: 2496

Re: Blackface Princeton Reverb clone, el84 reverb driver mod

Assume net plate volts of ~350 and 27ma that's ~9.5watts. I guess you got the 10watt tranny or still 3watt? With cold enough bias it will cold-clip if driven hard. I saw that happening on my '71DR so I heated it up somewhat. The later Twins did this. I don't know if the 2k2 makes it cold enough for ...
by jjman
Thu Nov 29, 2018 3:27 am
Forum: Fender Discussion
Topic: Questions about modifying 79’ Twin Reverb amp.
Replies: 42
Views: 3663

Re: Questions about modifying 79’ Twin Reverb amp.

The bad diodes and HT winding imply possibly shorted main filter cap(s.) Make sure the fuse is correct. All those went bad on an amp I worked on and the fuse was way to high to prevent it.
by jjman
Sat Sep 29, 2018 5:35 pm
Forum: Fender Discussion
Topic: 6V6 Red plates on Princeton Reverb clone.
Replies: 15
Views: 2449

Re: 6V6 Red plates on Princeton Reverb clone.

Looks like your meter is 10M input so those readings imply a bad coupling cap. You can check the other one to verify it shows lower figures, to confirm the diagnosis.
by jjman
Sat Apr 28, 2018 12:18 am
Forum: Technical Discussion
Topic: Can't figure source of 50v AC
Replies: 17
Views: 1114

Re: Can't figure source of 50v AC

How many ac volts measuring from plate to plate of the PI? How many from the Rp10/Rp12 junction to ground?
by jjman
Sat Mar 24, 2018 7:54 pm
Forum: Fender Discussion
Topic: Fender '63 Reverb standalone
Replies: 12
Views: 1565

Re: Fender '63 Reverb standalone

Could be some sort of parasitic oscillation that gets removed via probing. I would add a grid stopper to pin 2 as the 1st attempt. Make sure it's wired as a stopper and not a dropper.
by jjman
Sat Mar 24, 2018 7:12 pm
Forum: Technical Discussion
Topic: Diodes made in China vs USA
Replies: 20
Views: 1885

Re: Diodes made in China vs USA

If they don't meet the specs I won't be surprised at all. Not meeting the specs is often what China products are all about.
by jjman
Tue Mar 20, 2018 1:37 am
Forum: Fender Discussion
Topic: Blues Junior thoughts
Replies: 20
Views: 3070

Re: Blues Junior thoughts

1970+ SF models have this cap. At least the DR and TR. I suspect it might curtail some of the highest highs coming off the reverb. Some people install tone controls in reverb circuits.

There are many things to dislike about the subject amp but that cap is very low on the list for me, if at all.
by jjman
Thu Mar 15, 2018 11:55 pm
Forum: Technical Discussion
Topic: Lowering Bass
Replies: 9
Views: 772

Re: Lowering Bass

I agree with eliminating that mumbo jumbo on the input. If changing the leak to a 1meg you'll want to add cathode bias. You could then reduce bass by selecting an appropriate value for the cathode cap. Although a conversion may eliminate the need to reduce bass.
by jjman
Sat Mar 10, 2018 1:50 am
Forum: Fender Discussion
Topic: Ideas on 2 Reverb Send Controls
Replies: 4
Views: 988

Re: Ideas on 2 Reverb Send Controls

You could have both via a kill but it's an added switch and some additional loading on the live side when engaged.
by jjman
Wed Mar 07, 2018 4:21 pm
Forum: Fender Discussion
Topic: Ideas on 2 Reverb Send Controls
Replies: 4
Views: 988

Re: Ideas on 2 Reverb Send Controls

Sounds good to me. Or use a mixer and a normal post-return overall control. With no overall control, their could be more noise from the send and return stages since there would be no attenuation option after those. The mixer may allow for more send signal strength from less load on it given no groun...
by jjman
Wed Mar 07, 2018 2:07 pm
Forum: Technical Discussion
Topic: Gibson tremolo oscillator - where's the cathode resistor?
Replies: 9
Views: 695

Re: Gibson tremolo oscillator - where's the cathode resistor?

That one is placing a modulating signal on the grids of the output tubes via the PI transformer secondary. That pushes the bias up and down.
by jjman
Wed Feb 28, 2018 12:34 am
Forum: Technical Discussion
Topic: Stereo el84 se donor amp conversion
Replies: 12
Views: 1369

Re: Stereo el84 se donor amp conversion

Wouldn't the hum cancel in the same manner as a HB guitar pickup would? Those have 2 separate windings and poles. 2 single coil pickups can be humbucking if they have inverted magnets and are wired properly. If the PS voltage goes up momentarily (as with hum) both outer ends of the primary string go...
by jjman
Tue Feb 27, 2018 3:27 am
Forum: Technical Discussion
Topic: Bench Issue
Replies: 5
Views: 389

Re: Bench Issue

Sounds like a bad neutral on the power input to the amp. Either the outlet or inside the amp. Bad neutrals are unsafe. You could end up taking its place instead of the ground on the scope. Remember the ground on the scope is connected to the neutral at the house service panel.
by jjman
Sun Feb 18, 2018 11:43 pm
Forum: Technical Discussion
Topic: Random Acts of Stupidity
Replies: 9
Views: 694

Re: Random Acts of Stupidity

When I built my 1st amp it was about 23 years ago. It’s a 5F1 Champ circuit and there was only one problem. Huge hum that increased with the volume control. I had read about proper grounding but this chassis is so small I didn’t pay too much attention. And there were no websites related to amps back...
by jjman
Fri Feb 02, 2018 12:55 am
Forum: Trainwreck Discussion
Topic: small mods to make an Express pedal friendly
Replies: 4
Views: 1099

Re: small mods to make an Express pedal friendly

If you have the amp set low enough to not be clipping, it is already in pedal-friendly mode. The un-friendliness occurs if the preamp is clipping somewhere and certain effects are in the front and not a loop. In this case the loop has to be after the 3rd preamp stage since that clips. The 1st two pr...