blkface deluxe reverb startup problem

[yunger]

i recently did a filter cap job on a friends dr that he found in a lawnmower shed. i had to replace a few components and hardware also but it worked just fine. he's been playing out with it for a few months since the work and all of the sudden he is having problems. when he first fires up the amp and strums the guitar, there is a spatty, gated sound if anything at all. after about two minutes or so the normal sound kicks in and all is well.
i don't have the amp with me for testing right now so i can't measure anything yet. just wondering if these symptoms were typical of a common problem with older amps. otherwise i figure i'll start replacing all the old electrolytics.

[52 Bill]

...a spatty, gated sound if anything at all. after about two minutes or so the normal sound kicks in and all is well.

Check the tubes, check for cold solder joints and loose parts. Although anything is possible, this doesn't sound like a filter cap problem.

[yunger]

i finally have the amp in my possesion to work on it. the power tube sockets were loose and needed cleaned and retensioned. the startup problem is now solved although i noticed high plate voltages when biasing, 466 w/ the tubes in.
now i'm dealing with an unordinarily loud hum/buzz.
it hums w/ volume at 0>>>gets quiet at about 4(some kind of frequency cancellation??)>>>gets louder until about 8.5>>>quiets down again and then loud after 8.5 on the knob. with the reverb tank disconnected, turning up the reverb knob really increases the noise. i swapped in known good preamp tubes in the preamp stages and that didnt solve the problem.
fyi, i recently replaced a faulty power transformer, switched to a 3 prong cord and replaced all filter caps. my next move is to replace all electrolytics, and replace the diode and resitors in the bias supply.
anything else i should try?

[yunger]

if anyone wants to help me with investigating my noise problem here are the clues i've come up with:

-i have high voltages everywhere. 466 at power tube plates, 6.6 filament ( am i to understand that this is par for the course due to higher wall voltages?)
-pulling the first three preamp tubes has no effect on the hum.
-pulling the 5th tube, the vibrato tube, has no effect on the hum.
-hums with or without the guitar cable plugged into any input jack.
-hums with both volume pots at 0 and increases in volume as either volume pot is turned up. HOWEVER, various positions on the volume pots cancels the hum (wierd).
-pulling the 4th preamp tube, the reverb driver, kills hum volume but i can faintly hear humming.
-pulling the p.i. tube completely kills the hum.

if i were to give it an uneducated guess i'd say:
-the hum is coming from the preamp circuit, up to and maybe including the p.i..
-i feel like i have hum coming from both sides of the volume controls since they cancel frequencies.
-i don't know if the old bypass e-caps would cause this but they are on order so we'll see.
-maybe swapping filament leads at the lamp would help...doubt it though.

any and all suggestions would be greatly appreciated.
thanks

[MWJB]

High plate voltages in themselves are not a problem, use EH/JJ 6v6 tubes. NOS brands will be running way over design limits so you might want to try increasing plate current up to 30mA per tube, to bring the plate voltage down...you nearly always get 425vdc on the 6V6 plates if you do this...and/or try a 5V4 rectifier.

Heater voltages are fine.

Replace the electrolytics in the preamp as a start.

Swapping the filament wires won't change anything, if the problem was specifically the heaters, then the amp would still hum with just the power tubes...makes sense to check the condition of the 2x100ohm virtual centre tap resistors though.

Check grounded ends of components on the board (once you have replaced all the bypass caps) and look for significant resistance between them and the chassis ground. Pull the faceplate & control pots/jacks out and make sure there is no corrosion between chassis & brass grounding buss.

The 2 channels are out of phase, so it's not suprising that some settings cancel hum to some degree. This may suggest a component common to both channels as a culprit, like a shared ground or V2, pin 8 bypass cap?

I hope you upped the uf on the first (B+) filter cap, some BF Deluxes only had 16uf there, 32-50uf would be better.

[yunger]

thanks bill and mwjb for your replies.

the amp does have jj 6v6s @ 25mA right now. i'll try 30mA.

as for the filaments. there is no virtual ct. just an actual ct from the tranny.

i'll do what you suggest on the bypass caps and ground plate. i agree that this will probably solve the problem.

ignorantly, i replaced all filter caps with 16uF per the schematic. i also ordered a 50uF 50v for the bias cap rather than 100uF 100V. another thing i noticed was the p.i. plate resistors are both 100k rather than 100k/82k. i wonder if these things are worth another order to AES. i hate paying so much shipping because i didnt order all the parts the first time.
thanks

[olddawg]

Did you have the hum problem immediately after replacing the PT, or did the humming start later? have you tried chopsticking the wiredress. Is the PT grounding scheme the same as the original?

[yunger]

Did you have the hum problem immediately after replacing the PT, or did the humming start later? have you tried chopsticking the wiredress. Is the PT grounding scheme the same as the original?

that's a good question. i'm embaressed to say that i just noticed there is no center tap for the 6v winding on this new mojo power transformer. i guess i'd better create a virtual ct for that. i'm suprised that the power tubes aren't humming away also.

[olddawg]

that's a good question. i'm embaressed to say that i just noticed there is no center tap for the 6v winding on this new mojo power transformer. i guess i'd better create a virtual ct for that. i'm suprised that the power tubes aren't humming away also.

That is a very common senario with a replacement PT. Was the old PT the reason the amp was in the lawn shed? BTW IMHO Mojo products are highly overated and overpriced.

[yunger]

well there you have it!! quiet as a mouse. i had hooked up the power transformer until i ran out of wires and didn't even think about creating a virtual center tap. i'm still going to replace those electrolytics. since my friend already likes how the amp sounds and it is quiet now i think i'll leave the filter caps as is for now. thanks for your help 52 Bill, MWJB and olddawg!

[yunger]

That is a very common senario with a replacement PT. Was the old PT the reason the amp was in the lawn shed? BTW IMHO Mojo products are highly overated and overpriced.

yeah, someone used the amp as a p.a. system and blew the pt. my friend was using it for spare parts and thought it was just an "old peice of junk" until he started getting into tube amps...ha. he ordered the new pt and had me put it in and get the amp running for him. now he thinks it's the best sounding amp in the world. he's gonna be tickled pink now that the noise is gone.

[MWJB]

"another thing i noticed was the p.i. plate resistors are both 100k rather than 100k/82k." What value is the PI tail resistor? 27K or 22K? If 27K then 100K for both PI plates is correct AA763 value...even if you have a 22K tail the amp will work well enough with 2x100K...you say your friend likes the tone as is, so why change it.