The Amp Garage

People like Greg Germino and Chris Merren do wonderfully well at replicating Marshalls from specific periods, so it is possible to source highly "accurate" components. Transformers are key of course. And coupling/tone caps. Filter cap values are all over the place in classic Marshalls with some of t...

Forgive my ignorance: what's a PI cathode follower?

When is the last time your Hickok was calibrated? Industrial testers measure gm in incredibly goofy ways. The signal to the grid is derived from unfiltered rectified wall voltage, plus another AC component and isn't measured, just assumed to be correct. If the line voltage is off, or the rectifier t...

Heyboer. #996

The screws are probably reverse threaded so they'd be "righty loosey" counterintuitively.

The screen in a 6V6 can't pull all that much current before it melts, so tonal differences from screen sag with varying resistance will likely be small. Tweed amps without chokes typically used 10K there! But the intent wasn't to sag the screens, it was to keep variations in plate supply voltage fro...

The EA10 schematic shows 59V difference between plate and screen grid across a 5K resistor, so depending on the tubes you could see 85V across the 6K. That implies that the screens are idling at almost 2W, so if those are actual 5881s (3W screen max), they may start to light up when signal is applie...

There seems to be less and less GA-77RV as we go along. It looks like there's no choke. I think I can make out a wirewound off one of the output tubes in the original pic. Probably between plates and screens. The 77 was fixed bias and had separate tone controls for each channel. The EA-10 had three ...

A 20uF main filter might be a bit light for a pair of 6L6s (unless you like that underfiltered ghost note thing). The 77RV gets away with it because the choke is first in line. Does yours have a choke? Where is it located electronically?

If epiphonewiki is correct about this wanting 2x6V6, this probably matches closely to the BA-15RV, but with a 15" speaker and 12AX7s instead of 6EU7s. PI is cathodyne, so the reverb is driven off half a 12AX7 (V2b maybe) and recovered with other half of the 12AU7. According to Blue Book, Gibson only...

The fact that the B+ goes UP (and fluctuates) implies that the power supply is losing its load intermittently. Check all B+ connections, including OT leads; check the screen supply (resistors and connections) choke leads and bypass the choke as Stevem said. Also check the bias supply, especially the...

It is 4ohm for each jack, but when only the first jack is used, the OT's 4ohm tap is connected; when the second jack is also used, the 2ohm tap is connected.

That OT has two secondary taps. Check the wiring to the jacks (one is a weird switching jack) to see if the amp was set up to use the yellow-green 2 ohm tap with the 4x8ohm speakers.

This is the problem with almost everything said about tube rectifiers: that they will drop such and such a voltage. Under what conditions? If you look hard enough, you will find voltage data for half load versus full load and have to do the math in between. Amplifiers are dynamic.

Possible? Yes. Unpleasant? Maybe. There are cone cry artifacts sometimes mistaken for IM distortion, but mostly it's a filtering or ground issue.