Guild Master Amp mod. 99-J: opinion and comments wanted

[slidincharlie (Carlo P)]

I have just bought a late 50's Guild Master Amp mod. 99-J. Two 6L6 with tremolo and an original Jensen P12R.
I'd like to hear any kind of comment, opinion and thought about this amp or similar Guild models of that era. Especially comments on their tone and overall quality.
Do you know any web source with info on such amps?
Thanks,

[Enzo]

I rebuilt a Guild Master Amp from that era a while back. It was different though, it had SIX 50L6 output tubes, and no power transformer. One small 12v filament transformer for some of hte small tubes was all.

Your version sounds like the one they have on file at SChematic Heaven.

[slidincharlie (Carlo P)]

Your version sounds like the one they have on file at Schematic Heaven.

Yes, I think you're right.
I can't check now, it's still in the mail.

[blueloonrex]

99-j masteramp

I have a '59 model & would not trade it for anything. It has tone to die for.

[slidincharlie (Carlo P)]

Hi blueloonrex,
I have recently restored mine - it had quite a few things that needed to be fixed or upgraded. There is a thread of mine in Ampage concerning the restoration of this amp and the problems encountered.
Now it has two 6V6's (the 6L6's did not meet the power tranny specs).
The tone is nice, tweed-like, but the volume is WAY lower than expected; I would say that it compares to a Fender Champ.
How does yours sound, volume-wise?

[loudthud]

There is a Guild Amp Forum here:

http://letstalkguild.com/phpBB2/viewforum.php?f=7

Additional schematics can be found here:

http://www.prowessamplifiers.com/schematics/misc/

I got interested in Guild amps last week when a guy on Talk Bass posted a picture of an amp he just scored. The late 60's early 70's bass amps look interesting. Voltage doubler power supply, feedback around preamp stages, effects loops, the Quantum Bass amp even has a voltage regulator for the screen supply. 100W from a pair of 8417's. 3 meg tone pots are a little weird. They look to be well designed and used state of the art (at the time) RCA tubes. Some models came with JBL speakers.

Hey Enzo, the first amp I ever built had a 50L6. It's a wonder I survived!

[Enzo]

Did you have a 35Z5 to go along with it?

[loudthud]

I think it was the 35Z4 and a 12SQ7. I was in 7th grade!

[Enzo]

The "All American Five" before they went miniature to 50C5 and 35W4.

I admit I was not hip to isolation transformers back then either.

When I went off to college I got a brand new AA5 table clock radio for my dorm room. Worked OK. One day for whatever reason it tried to wake me up, and I was not coming around in response. I just perceived it as some annoying noise the radio was making. SO in my half-asleep state I wrapped the radio up in a blanket to stifle the noise it was making (the alarm). I woke up a couple hours later and found the radio wrapped in the blanket, with just a dim recollection of having done it. The plastic radio shell had melted like a Dali painting. It still functioned, but the case was all collapsed and drooping down over the things inside. Like a sheet over unused furniture. I don't know what I was thinking.

[capnjuan]

The late 60's early 70's bass amps look interesting. Voltage doubler power supply, feedback around preamp stages, effects loops, the Quantum Bass amp even has a voltage regulator for the screen supply. 100W from a pair of 8417's.

Hi Loudthud; I recently rebuilt the power supply in a Guild Thunderbass head; 3X12AX7, 1X7247, and 2X8417. Because there are so few 8417s out there, I tried to build it back as a 6L6 amp. Using the 6L6 schematic, I changed several resistors and a minor change in the bias resistor/voltage. Given the similarity in power supplies between the 8417 and 6L6 models, I think I wrongly assumed that the transformers were the same for both models but I never got the B+ past 400volts, the rating for the 8417s, not the 460volts in the schematic for the 6L6s. The plate / screen voltages are in the same proportion as the 6L6 design, just not as high. Even though it doesn't take complete advantage of the 6L6s higher voltage handling, it's a sweet, good-sounding amp; a little 'clinical-sounding' by my tastes for guitar but a punchy, toneful bass amp. John

[loudthud]

The schematic I found for 8417's has a B+ of 560V with 300V on the screens. That's what the RCA tube manual says will give 100W with a 4.2K OT. They used a 6GF7 as a voltage regulator, that's what RCA used in their 50W power amp from the tube manual circuits section. There could be many variations in Guild amps over the years but I would think any design with 8417's would have a higher B+ than 400V. The 6550 is close to the 8417 but has lower transconductance and higher saturation voltage.

[capnjuan]

Hi Loudthud; of course we're talking about 2 different amps; the Guild Quantum, Superbird, and Thunderstar - some of which (early ones?) were 8417 amps - used the 6GF7 screen regulator. The Thunderbass doesn't. Below is a pic from the Thunderbass 8417 schematic which indicates 400V on the plates and 350(?)V on the screens. My finished unit has 6L6s in it and, built part-for-part like the Guild Thunderbass 6L6 schematic that I have, generates 400V on the plates and 360V on the screens.



There's the chance that I could have made a mistake but without going into too much detail, the two major dropping resistors (for 6L6s) that went into the rebuild were smaller values than the ones (for 8417s) coming out; was thinking would have more voltage. Assuming I didn't mess up, the only explanation for getting the same/similar voltage would be that the 8417 and 6L6 models use different transformers, no? CJ