The Amp Garage

With the surgistor jumpered, the B+ at cold startup goes up to about 510v for a sec before settling down to 465v. I'm using 500V F&T caps. Once the tubes warm up the primary side of the transformer is pulling just shy of 800 mA.

Thanks Steve, there's no standby switch. I could replace the mains switch with an off/on/standby 3- position switch but the original Vox switch with the square-ish plastic bat is part of the look of the amp. Series connection if filters isn't really an option without messing up the layout of the amp...

Hey everyone. I have and AC50 MKIII on the bench with a Surgistor that is acting up. For those unfamiliar, this is a mechanical device intended to replicate the soft start of a GZ34 in a solid state rectified amp. Here's a link with a nice look at what I'm talking about. http://www.voxshowroom.com/u...

I picked up RG Keen's "Vox Owner's Safety Net" recently for some background info on a Thomas-Vox Beatle that came across the bench. The book has a clear breakdown of the various circuits used in the Thomas Vox amps. What I wasn't expecting however was a really great section on debugging transistor c...

Looks like 10k and 470r on the tail of the PI rather than 22k and 100r. Maybe the mid and bass caps in the tone stacks are both changed to .022? The mixer resistors going into the PI look like maybe 47k?

Is that a 100k negative feedback resistor?

Wouldn't the B+ be a bit high using the spec brown note PT with no tube rectifier?

I put a Celestion 10 Greenback in my Princeton the other day and was surprised at how good it sounded. The UK-made 10" Greenback is a lot like it's 12" sibling. Woody and spacious-sounding, seems considerably louder than the vintage Jensen Ceramic that was in there. I'm leaving the G10 in for now. I...

Phil B seems to like somewhat under-rated iron in the power supply. The 5B6-based amp used Hammond and Classictone Deluxe Reverb replacement power transformers with 6L6GC's. I was talking with a friend who is a pro Steel player with really good ears about how good the old Princeton Reverbs with the ...

Looks like JJ KT88's in the first picture (round holes in the plate) and JJ KT66's in the 2nd picture (rectangular holes in the plate).

For what its worth I've tested (literally) thousands of JJ preamp tubes on an Amplitrex and and the ECC83-types come in within 10% of spec about 90% of the time. The higher-current tubes like the 12AT7 and 12AU7 seem a little harder to get dialed in to dead-spec, though they are most often within th...

The 12AT7 is unusual for the 12A_7 family in that the manufacturing tolerances for the tube as-designed appear to be incredibly wide. Check out the "Design Minimums and Maximums" under the Electrical Tests section in this Raytheon data sheet: https://frank.pocnet.net/sheets/138/1/12AT7WA.pdf Accepta...

What parameter does your tube tester test? 55% of what? Spec transconductance, emissions, gain? What tester are you using?

I'm late to the party on this one but if you check the schematic for this amp note that even though it calls for a 12AU7/ECC82 in V1, the side of the dual triode used for the Brilliant channel appears to be configured for an ECC83/12AX7 (100K plate resistor, 1.5K on the cathode). I heard an original...

I don't know if there is a Holy Grail of tube testing. This looks great though:

http://www.roehrentest.de/EnglishInfo.html