Recent Posts

51

Tube Amp Building - Tweaks - Repairs / Re: Reverb bleed at 0 on RVB pot on a '65 blackpanel Fender Vibrolux

« Last post by Sansteeth on March 10, 2026, 03:17:19 am »

Thank you sluckey for taking the time to help me!

This sounds like a non-issue to me. Teach the customer how to use the footswitch.

  you are not wrong

Connect a gator clip lead between chassis and the wiper of the reverb pot. Does this kill the reverb?

Nope, which makes me think it's happening somewhere else....

Also replace the 25µF cap that connects to pins 3 and 8 of V4.

That also doesn't kill the reverb 

52

Tube Amp Building - Tweaks - Repairs / Re: LOW Output from 50's GA20

« Last post by Yeatzee on March 10, 2026, 12:46:59 am »

Welp, decided to test the OT primaries and secondaries. Primaries seemed normal about 212ohm each side to CT, plate to plate 423ohm. Secondaries, injecting 3.4 volts in via my variac to the 8ohm tap I got a voltage drop of about 1 volt so it's pulling current. DANGIT.

53

Introduce Yourself / Hello, from Connecticut!

« Last post by EL34 on March 09, 2026, 10:21:59 pm »

Welcome to the Hoffman Amps forum

There are many different boards on this forum for different topics.

The Tube Amp Building - Tweaks - Repairs board is where you can post amp questions.

That board is listed here.
https://el34world.com/Forum/index.php?board=13.0

Thanks,
Doug Hoffman

54

Introduce Yourself / Hello, from Connecticut!

« Last post by Moose on March 09, 2026, 10:21:59 pm »

     Hello, I’m a retired Aircraft Mechanic who just wandered into tube amps, mainly by finding Uncle Doug’s excellent How Tump Amplifiers Work series. I watch Uncle Doug, Yateze Guitar, Psionic and The Guitaroligist. I’ve been lurking here and at Robrobenette’s site. Any rate as a fledgling guitar/bass player, who owns a Vox AVDT 30 and a Bass Cube, I decided that I’d like to build something Fender. I decided to join so I can ask and get some Info on getting DIYLC up and running on my old iMac using MacOS Sierra 10.12.6 |. It keeps flagging an error “Could not open open file… Since I’m not very computer savvy, I’m stumped. Hopefully someone can send me over to a posting about trouble shoot ancient Macs running DIYLC. Thanks in advance. Moose.

55

Tube Amp Building - Tweaks - Repairs / Re: LOW Output from 50's GA20

« Last post by Yeatzee on March 09, 2026, 08:30:26 pm »

Adding some more voltage readings for reference. Seems quite low, but I have nothing to go off of. 1meg resistors dropping the voltage a ton for the preamps, but as designed?

56

Cabinets-Speakers / Re: Options on reparing/ replacing integral baffle?

« Last post by vintasonic on March 09, 2026, 07:44:08 pm »

another pic

57

Cabinets-Speakers / Re: Options on reparing/ replacing integral baffle?

« Last post by vintasonic on March 09, 2026, 07:42:38 pm »

Apologize for not responding sooner, hella busy

I would not give up on this idea. If you use a glue like Titebond it would need to be clamped

So my plan is still along the lines of repair as opposed to replacement, the baffle is separating at the top but is still kinda solid and the speakers help hold it together as well. I will use either Gorilla Wood Glue or Original Gorilla Glue; I have had a lot of luck lately repairing with the Original, it's polyurethane based and fills gaps nicely but the Wood glue might flow between the layers better.

Would it be easy to cut the entire baffle out?

I could but it's more work than I want to put into it at this time.  I could also possibly cut away most of it and leave about an inch of the existing baffle in place to attach a new one. But that also means getting or creating a new baffle  -  the owner is OK with repair if I can make it work

So I think I will start with gluing and clamping and see how it goes.  I have done a bunch of cabinet repair lately, it seems feasible and a lot less work.  This amp is road worn and not going anywhere soon, so repair may be good enough.  Glad to hear your opinions before I start & I'll post after it's done.

Thanks

58

Tube Amp Building - Tweaks - Repairs / LOW Output from 50's GA20

« Last post by Yeatzee on March 09, 2026, 07:41:59 pm »

Getting Deja Vu with my own personal GA40 I had a similar issue with and never got around to solving it. Have an early GA20 in, matches this schematic:

https://www.prowessamplifiers.com/schematics/Gibson/ga20.pdf

The amp was serviced somewhat already before it came to me, with replaced electrolytics and a 3 prong. The rest was largely untouched. Dimed the amp was barely bedroom volume. I measured the output wattage and it was putting out ~0.6w (2ishvac into 8ohm load) clean. Yikes! Here's what I've tried and done so far:

New rectifier, no change.
New power tubes, no change.

Replaced all of the waxy coupling caps with Mallory 150's, lowered the bias resistor to 150R from 220R, dropped the screen resistor from 10k to 2.2k. This got me up to a whopping 2.7vac clean into an 8ohm load for .9w. Really rocking! Same for both channels.

Here are some reference voltages with all of the tubes in the amp:

Plate to Cathode - 262vdc
Plate Current - 36mA
6v6 Plate Voltage - 275vdc
Screen Voltage - 255vdc
5Y3 Output B+ - 285vdc

Bias is hovering around 90% plate dissipation. Screens were crazy low (210vdc) before I reduced the resistor from a 10k to a 2.2k. I've not run into an amp that ran the screens so low compared to the plates so I decided to just experiment and get it closer, still a big drop with the 2.2k. Obviously that wasn't the fix either way, more of an aside than anything.

So where to next? I've been at it for hours so taking a step away, and hoping you guys can inject some new ideas as to what I'm missing! Amp otherwise sounds great, just low output. Worst case OT. Also don't have a spare 6SL7 phase inverter so may need to order one to test if that could be it.

59

Tube Amp Building - Tweaks - Repairs / Re: Revisiting Sears Silvertone 1484/85 Piezo Tanks.

« Last post by DummyLoad on March 09, 2026, 04:47:49 pm »

Very glad to see this work done. The original Silvertone reverb tanks obviously being a weak spot. I wonder if one of those blue plastic tanks I see in pedals would work to fit it inside the original amp head...

Thanks!
I'd go with a tank like in the link below, if that's what I planned to do; metal enclosure construction so better shielding. The issue with the Silvertone cabinets is the location of the PT being located at the center of the chassis, mutual inductance from PT to the reverb tank is an issue, I did test with a type 8 tank and was not satisfied with the hum level. Also, these mods work best with a type F tank, 1475Ω input Z - Those Blue tanks have 800Ω as the highest input Z available and better suited for SS drive/recovery ckts. 


https://www.amplifiedparts.com/products/reverb-tank-mod-mn-8eb2c1b-medium-decay-3-spring-mini

--Pete

60

Tube Amp Building - Tweaks - Repairs / Re: Revisiting Sears Silvertone 1484/85 Piezo Tanks.

« Last post by passaloutre on March 09, 2026, 03:56:26 pm »

Very glad to see this work done. The original Silvertone reverb tanks obviously being a weak spot. I wonder if one of those blue plastic tanks I see in pedals would work to fit it inside the original amp head...