Tube Amp Archive

Auke wrote: ↑Thu Oct 16, 2025 5:55 am I don't know about this specific transformer but had good ressults with other transformers I bougth from them.

+1. The express PT is good

As the current Output of any given PT starts to reach its limit then the tone of the amp will start to change. The point where the power supply voltage starts to sag is where you have maxed out the current available from the PT. This is also the point where the filament voltage will sag with the re...

Yes, those 100R resistors are screen stoppers. Because of their low value they don't do much for screen current limiting. From tube literature (e.g. RDH4) it is known that directly paralleling power tubes invites RF oscillation. The screen stoppers provide some tube separation. The 400R in the powe...

There are several factors that affect overall tone. When a transformer is swapped out, you need to confirm that the output tubes are biased the same and the Voltages are the same throughout the amp. One often overlooked Voltage is the Heater Voltage. Don't believe me ? Try this experiment. Connect ...

Look a the big shared screen resistor that reduces voltage and current. Looks like 400R here Roe: Peavey uses a similar 400 Ohm resistor in many of it's amps. But unfortunately, that does not explain the inconsistent usage of the 100 ohm resistors. Thanks. no wait a second. you're discussing the in...

Look a the big shared screen resistor that reduces voltage and current. Looks like 400R here

1) If using a bridge rectifier with the artificial centre tap, is it better to keep the Standby switch where the Traynor currently has it where it lifts the ground from that Bridge Rectifier? Or have it after the rectifier and first Filter caps? neither, lift both ac wires instead Is there a second...

1) If using a bridge rectifier with the artificial centre tap, is it better to keep the Standby switch where the Traynor currently has it where it lifts the ground from that Bridge Rectifier? Or have it after the rectifier and first Filter caps? neither, lift both ac wires instead Is there a second...

bcmatt wrote: ↑Wed Mar 12, 2025 9:05 pm 1) If using a bridge rectifier with the artificial centre tap, is it better to keep the Standby switch where the Traynor currently has it where it lifts the ground from that Bridge Rectifier? Or have it after the rectifier and first Filter caps?

neither, lift both ac wires instead

All your voltages are low as compared to the mojo schematic. Look at the attached schematic. Post the measured B+ voltages at points XA, XB, XC, XD, and XE. Also measure the resistance of the three 10K dropping resistors on the B+ rail. move the bias supply before (not after) the standby switch! Th...

One odd thing that I think I understand: the bias pot swept from ~8 mV to ~50mV. Biased at 41 is a little past 3/4 of the way. To get it centered better I probably need to raise the 28k in series with the bias pot and maybe increase the dropper from 100k to 120k. I don’t know exactly what the schem...

345v at idle seems right for the late 70s amps. I've lowered the voltages to around 320-5v, using the 245v primary with 230v wall voltage (and a termistor, no standby switch, and ss rectifier) Thanks, that is very helpful. Regarding your rectifier, did you pad the secondaries with some resistance a...

amp is good for clean tones and modest distortion. my old ac30 will blow it off stage anyday - way more aggression, punch, tightness and better distortion (although it sounds more unforgiving) Interesting. May I ask what the voltages are in your old AC30? Or what they should be for a JMI 4xEL84? I ...

Thanks Roe. I would be interested to know what the EL84 plate, screen, and cathode voltages look like. the voltages are on the uploaded schematic. it is like an ac30 except for slightly lower voltages and lower Z (3k3-3k8 probably) Thank you, Roe. Yes, I saw that. Voltages look similar to a Rocket ...

Colossal wrote: ↑Wed Sep 24, 2025 11:04 am Thanks Roe. I would be interested to know what the EL84 plate, screen, and cathode voltages look like.

the voltages are on the uploaded schematic. it is like an ac30 except for slightly lower voltages and lower Z (3k3-3k8 probably)