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91

Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 14, 2026, 10:03:57 am »

Sluckey, it's bands appear to be Brown, Black, Brown with a Gold band for tolerance. So that's 100 ohms.  It's pretty big, physically, and on one of the schematics, not this particular amp because there isn't one that I've been able to find, it's shown as a 3W resistor.

But in looking at the Voltages, I'm concerned about the C+ since it's a 450V cap.  Both the A+ and the B+ are 2 caps wired in series.  The A+ is two 350V caps in series for 700, but I can't read the B+ voltage because its installed face down to the PCB.

D+ is a 22uf/450V cap and its reading 420VDC.  So it may be okay.

The PT's secondary reads correctly at around 380VAC, so when rectified its around 535VDC.

When I checked the voltages with the current limiter in-line, the voltages read about 100VDC lower.  I know they would be lower in a restricted mode, but I don't know if 100VDC indicates anything.

So there's a lot of voltage going into this amp's circuit.  An amp built in 1997-if that indicates anything.

92

Introduce Yourself / Re: Glad to be here!

« Last post by rake on April 14, 2026, 09:55:08 am »

Welcome
(former New Englander here)
Doug's the boss here but there's a lot of other real smart guys here ready to help too,
but, they will get a bit cranky without a schematic so,  post a schematic! LOL!

93

Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by sluckey on April 14, 2026, 09:23:55 am »

In addition there is a 100 ohm dropping resistor between A+ node and the C+ node that was reading 10K!!  So I replaced that.

I'm pretty sure that resistor is supposed to be 10K. The very high voltages at node C and D seem to support that guess.

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 14, 2026, 08:58:58 am »

Turns out there was another issue that I was able to diagnose and fix.

There is a 62K resistor in the bias circuit that I found was blown.  So I replaced that.

In addition there is a 100 ohm dropping resistor between A+ node and the C+ node that was reading 10K!!  So I replaced that.

And finally the C+, 100uf/450V, filter cap was shorted.  So I replaced that.

I think all of these things were related to what I believe was a blow out when one of the output tubes with the broken locator pin was probably inserted in the wrong position.

Now I've got it running.

Here are some numbers for you. It's running 6L6GC's. The numbers sound high, but maybe that's just because it's a higher powered amp that I'm used to dealing with.

Plates 494VDC
Screens 491VDC
Grids -50.1 VDC
Bias 43.1/40 ma
A+ 497VDC
B+ 491VDC
C+ 492VDC
D+ 420VDC

I guess that's about it for this one.  Thanks for everyone's help.

95

Tube Amp Building - Tweaks - Repairs / Re: Newcomb E-17 "guitarified"

« Last post by Jennings on April 14, 2026, 07:11:06 am »

Well, I have some great news...I did some "real" speaker testing, and still thought the amp was lacking.  Remember it's currently virtually stock, except I've removed the NFB.  And then it hit me!  That 6SC7 is grid biased as stock in this circuit!!!  Silly me had removed the cap when I installed the 1/4" input jack...DOH!!!  So input cap re-installed and tested again and the amp is much livelier and sounding great.  I don't think I need to really change anything else now.  Thanks for the input folks.  I've drawn and attached a final schematic for the records, in case anyone finds this thread in the future.

96

Introduce Yourself / Glad to be here!

« Last post by EL34 on April 13, 2026, 10:11:35 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

97

Introduce Yourself / Glad to be here!

« Last post by wyreit on April 13, 2026, 10:11:35 pm »

Hey guys!

 Hi everyone! In the middle of my first build, so I figure now is a good time to say hello

So far, so good, but I am sure I will have a question or two.

About me

- Dad to some great kids
- Been a software engineer for the last decade, but was in IT or tech-related fields for the last 25 years.
- never really played guitar or used a soldering iron until last year

how I got here

-Last year, I realized that I needed a hobby that was a complete mental break from anything computer-related.
-I started learning guitar and music theory for the first time. The last instrument I held before that was a flutaphone in 3rd grade.
- 3 months later, while googling for guitar, I clicked on a link to Rob Robinette's site on how tube amps work.
- Something clicked, and I just fell in love with it.
-Started by building a few pedals to get my soldering down


For my first amp build

Circuit: Modified Micro Bassman From Rob Robinette (turret build)

Mods
-Switched Bright volume
-3-way pre-amp cathode bias switch for V2
-Lamar type 2 MV

Chassis:

Blank flat face Fender Deluxe Reverb (~22x7x2)

-I went with a chassis with tons of room that would still fit a 1x12 cab.



Taking my time, but already thinking about the next one... 


Look forward to getting to know you all, thanks again!

98

Tube Amp Building - Tweaks - Repairs / minor Major 7 combo

« Last post by CascoSieg on April 13, 2026, 08:11:14 pm »

A couple of years ago I started reading through a thread on this forum that focused on the Marshall Major, and whether and how one might capture the vibe of the original 200W item in a smaller scale. https://el34world.com/Forum/index.php?topic=17548.0
After working out and abandoning an ideal design/layout (due to funding constraints and the fact that I didn't think I actually "needed" another amp) I decided to use a donor amp that I was unable to sell, and build a combo version of the minor Major (my seventh build, as luck would have it!), salvaging whatever I could, namely the chassis, cab, speaker, switches, various and sundry components. Here's a link to a few pics if interested:
https://photos.app.goo.gl/d4tNKLT32JYsRQUS8
It turns out I DID need another amp after all! It's my new fave, especially with the Tele - turned out great, even in the odd format with all the constraints and compromises. I've never heard a Major in person, let alone played one, but this amp has a kick, an immediacy, massive bottom end and a clarity throughout the range, that I haven't experienced in any amp - Wow!
There's one odd thing, which may or may not be an issue: The plate voltages for all the small tubes came out pretty close to those listed in the Marshall Voltages spec that was floating around, except for the 12AU7 driver: mine are way short - 210v (leaving 9.1v at the cathode) instead of the spec'd 310v (leaving 15v). My B+ at that node (349v) must be way less than the Major, or something else I don't understand is going on. Given that (1) I'm only swinging current into two (instead of four) KT88's, and (2) 349v exceeds the typical max plate rating for the 12AU7, I'm wondering whether this is something that even in principle needs solving. In other words, does anyone with intimate knowledge of the Major think tweaking anything about the drivers (that would be possible in this amp) will make this amp behave more like the original?  FWIW the KT's are pushed plenty hard enough into overdrive for my liking, but as I've said, I've never heard a Major in person.
Thanks all, for the inspiration and guidance!

99

Tube Amp Building - Tweaks - Repairs / Zener Bias "Trick" for Cathode Biased Amps

« Last post by jbrrrrr on April 13, 2026, 04:30:49 pm »

Hello, amp tinkerers and tube zealots,

I stumbled across this chunk of a schematic somewhere, but I can't remember the source - it looks like this is a technique used in cathode biased amps with a shared cathode resistor, in order to control the amount of stiffness (or lack thereof) by clamping the cathode voltage at the limit set by the parallel zener(s).  See attachment.

Anyone have any experience using this zener trick as a means to control the 'feel' of a cathode biased output section?  It looks to me like in the 3rd schematic on the right of the 'Zener_bias_trick Squash.gif' file, if you wanted to make it switchable, you could maybe use a DPDT to ground the second series zener with one pole, and parallel in another value for the cathode resistor to make sure the bias sits appropriately in either position using the other pole of the switch.  I could see that being a more impactful modification than just increasing the bypass cap value for a stiffer feel.

Would anyone mind verifying if I'm understanding this correctly?

Regarding any other references to this kind of idea, it looks like tubecad.com might have been an early source for this - Broskie wrote about it here, in the second article on this blog post from 2007.  Paul Ruby is also credited in doing something similar but everything I've read about it seems targeted towards removing "buzz" or crossover distortion, specifically from EL84 tubes. 

100

Tube Amp Building - Tweaks - Repairs / Re: Bias Reader Built, works great, and revealed an issue.

« Last post by acheld on April 13, 2026, 09:54:21 am »

"But, we have AI now . . ." 

Quote from a 19yo, a good kid otherwise.