Search found 722 matches

by jjman
Sun Oct 15, 2017 2:08 pm
Forum: Technical Discussion
Topic: Help with switchable cathode resistor
Replies: 13
Views: 207

Re: Help with switchable cathode resistor

"B" looks better since there is no chance for a period of time with the cathode-path "open." But I'm thinking both would cause at least some "pop."
by jjman
Tue Sep 26, 2017 11:54 pm
Forum: Technical Discussion
Topic: mixing resistors
Replies: 12
Views: 287

Re: mixing resistors

I recently converted an amp from a pre-PI mixer (pot) setup to injecting into both PI grids. The “channels” (actually a parallel loop situation,) are out of phase. I had a phase flipping tiny transformer on the loop side with the mixer-pot setup, but I wanted more potential post-return loop signal. ...
by jjman
Sun Sep 24, 2017 10:02 pm
Forum: Technical Discussion
Topic: Problem with biasing Twin reverb type amp
Replies: 20
Views: 440

Re: Problem with biasing Twin reverb type amp

Placing the meter in the current path, as THAT probe does, means the meter's internal resistance skews the measurement. Even a quality meter can have that issue. Transformer Shunt Method also has that issue. Using the transformer-voltage-drop method or the cathode-resistor-voltage-drop method avoids...
by jjman
Sat Sep 16, 2017 3:37 pm
Forum: Technical Discussion
Topic: Supro s6424 build
Replies: 20
Views: 614

Re: Supro s6424 build

For both I would try the 1megs alone 1st. Then add the resistor if it doesn't seem right.
by jjman
Sun Sep 10, 2017 5:30 pm
Forum: Technical Discussion
Topic: Bias tweaker or "touch"
Replies: 6
Views: 322

Re: Bias tweaker or "touch"

I'm not understanding why that adjuster would affect the idle current with the LFO off. Maybe some DC is leaking thru the LFO coupler?
by jjman
Wed Sep 06, 2017 12:13 am
Forum: Technical Discussion
Topic: Output impedance tolerance?
Replies: 5
Views: 194

Re: Output impedance tolerance?

For tube amps it is safer to use a speaker with a "too low" impedance vs a "too high" impedance. For solid state is it the opposite. The risk depends on the output tranny and the amount of signal pushed. There is a consensus opinion on the internet that it is safer with a vintage Fender than a vinta...
by jjman
Wed Jul 12, 2017 11:45 pm
Forum: Technical Discussion
Topic: What to clean a chrome chassis with, that won't remove the lettering.
Replies: 7
Views: 354

Re: What to clean a chrome chassis with, that won't remove the lettering.

I use Nu-Finish on crome chassis. Hand polish and buff. No wax or residuals AFAIK.
by jjman
Mon Jul 10, 2017 11:39 pm
Forum: Fender Discussion
Topic: Static in reverb channel of '74 Deluxe Reverb
Replies: 5
Views: 240

Re: Static in reverb channel of '74 Deluxe Reverb

My '71 had board conduction where the 5 leads join that include the wire to the V4A grid. Faint crackling. I added a standoff to float that node off the board. Pressing the vicinity with a chopstick revealed it. (I modded the values of the 10pf/3.3meg to 47pf/1meg for more signal.)
by jjman
Wed Jun 21, 2017 11:35 pm
Forum: Technical Discussion
Topic: Help with load line for LTP phase inverter
Replies: 20
Views: 856

Re: Help with load line for LTP phase inverter

Since you are not averse to employing SS devices to the PI you could check on using a Constant Current setup on the tail. I don't know if those force more balance but it's worth checking.
by jjman
Thu Jun 15, 2017 11:53 pm
Forum: Trainwreck Discussion
Topic: Express Startup Issues
Replies: 15
Views: 643

Re: Express Startup Issues

Double check the measured value of the cathode resistor on v1a. If it is high those voltages would move in that direction.
by jjman
Sun Apr 30, 2017 11:27 pm
Forum: Trainwreck Discussion
Topic: Phase inverter issue on express build
Replies: 1
Views: 271

Re: Phase inverter issue on express build

Is B3 supposed to be that high? Are the power tubes installed? Did you double check the plate resistor values with a good meter?
by jjman
Thu Mar 30, 2017 11:56 pm
Forum: Technical Discussion
Topic: New amp build from old pa - transformer question...
Replies: 24
Views: 996

Re: New amp build from old pa - transformer question...

Some use the CT as one end. Many say it's taboo. Most talk about an "air gap" being needed. I think the issue is "saturation." (A bad kind.) Don't do it unless you know how to employ it that way.
by jjman
Wed Mar 29, 2017 1:51 am
Forum: Fender Discussion
Topic: VibroChamp / Pro Junior Boxy; Suggestions?
Replies: 11
Views: 670

Re: VibroChamp / Pro Junior Boxy; Suggestions?

The trem on the homebrew provides the "ground" connection to a Larmar Post PIMV on a ~5E3~ output.
by jjman
Tue Mar 28, 2017 1:48 am
Forum: Technical Discussion
Topic: 1974 Traynor YGM-3 Guitar Mate Reverb
Replies: 17
Views: 855

Re: 1974 Traynor YGM-3 Guitar Mate Reverb

PR Trem is an AC signal thru a cap to the bias, so it pushes the bias up and down. I think all non-photo-resistor trems push and pull. Photos just pull down.
by jjman
Sun Mar 26, 2017 5:21 pm
Forum: Fender Discussion
Topic: twin reverb bias question
Replies: 28
Views: 1035

Re: twin reverb bias question

I made that mistake once. Very frustrating. The internets tell us to use the bulb for initial startups but fail to remind us that we must stop using the bulb for any quantitative testing.