Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: philschl on August 30, 2025, 04:00:58 am

Title: Pre-Amp Anode Voltage Ibanez Thermion
Post by: philschl on August 30, 2025, 04:00:58 am

Hello,

I'm working on an Ibanez Thermion. The power supply is limiting voltage to preamp tubes V1a (219V), V1b (160V), V2a (160V), V2b (302V).

I want to increase V1a to 280V, V1b to 200V, V2a 210V, V2b 380V.

Attached the schematic and the snippets with voltages (hope this is an acceptable format).

Reason for desiare to increase voltage is to reduce mush and fizz.

The amp is very similar to dual rectifier, the target voltages are from the dual rec.

What resistors in the power supply need to be changed to increase the voltages?

I'm comfortable doing repairs, but redesign is still beyond me.

Thanks!
Phil.

Title: Re: Pre-Amp Anode Voltage Ibanez Thermion
Post by: Lekky on August 30, 2025, 12:01:47 pm

There seems to be three series-linked resistors dropping the voltage for the C, D and E preamp nodes - R232, R238 and R239. Shorting one of them or replacing one or more with a lower value should bump the voltage of the preamp section a bit.

By the way, check the voltage rating of the coupling caps in the preamp before bumping the voltage. 600V or so should be OK.

Title: Re: Pre-Amp Anode Voltage Ibanez Thermion
Post by: philschl on August 30, 2025, 09:58:02 pm

Thanks Lekky, that's what I was hoping for!

Title: Re: Pre-Amp Anode Voltage Ibanez Thermion
Post by: philschl on September 01, 2025, 06:14:17 am

Just to follow up, I shorted as suggested the R's as in the picture with the results. Configuration B (V1B at 208V) actually did sound best, adding clarity and removing mush.

The other voltages across the circuit all measured within tolerance of caps, and the resistors seemed to pack it away without getting hot as well.