DeArmond (Rowe) Guitar Amplifier Model R25T Restoration

[misterc57]

This shows the bias supply on the left

Thank you for the schematics. Now to find a suitable power transformer, as mentioned perhaps a Fender PT will meet the specs. I may need to somehow mount the PT to the bottom of the cabinet as I will likely never find something to fit the chassis.

[misterc57]

It appears that there is no bias tap off the power transformer. Instead one is "created" off one end of the HV winding and its center tap. Confirmed?

In which case this changes the specs for the PT I am searching for?

Thank you

[Helmholtz]

The schematic calls for a PT having a 300V-0-300V secondary. I wouldn't go much higher than that.
I don't think this is a 45W amp, more like 30W.
Higher B+ might put the OT at risk.

[misterc57]

The schematic calls for a PT having a 300V-0-300V secondary. I wouldn't go much higher than that.
I don't think this is a 45W amp, more like 30W.
Higher B+ might put the OT at risk.

Many of the PTs are 360V-0-360V. Have been looking at this one on Reverb. https://reverb.com/item/60365186-sta...961-black-iron

Kim, have you had a chance to check the B+ on your R25T PT? He told me he has 485V on the plates.

[Helmholtz]

Sorry, I misinterpreted the schematic.
From the tube voltage table, PT secondary voltage should be 340-0-340.

[misterc57]

From the tube voltage table, PT secondary voltage should be 340-0-340.

Would 360-0-360 be too high V?

Aside from the rectifier tube I note that the V table is slightly different on each schematic.

Thank you

[g1]

The only difference I see is V1, and the 6L6's being cathode biased on one version. Otherwise the plate voltage for 6L6's are the same, and both versions rectifier tubes show 340-0-340 AC.

[khmassey]

Interesting that the tables don’t match. I got the blurry one from one of the admins for the DeArmond site. Don’t know why they haven’t posted it on their website. So today I measured 760 vac on the PT secondary with 123 vac wall voltage

[khmassey]

So to me that transformer listed on Reverb looks like a good match. Not to mention the price is very good.

[misterc57]

The only difference I see is V1, and the 6L6's being cathode biased on one version. Otherwise the plate voltage for 6L6's are the same, and both versions rectifier tubes show 340-0-340 AC.

Yes interesting there are two versions. Mine is grid biased.

khmassey, I believe yours is also grid biased?

Thank you all.

[misterc57]

Regarding that PT on Reverb. I'm wondering about the 5V winding 2A limit being right at the GZ34 1.9A. The 6V winding of 4A looks good.


He spec'd as follows

input 117 VAC

outputs:
red or pink pair 720 VAC @ 60 mA, center tapped with red/yellow (other sellers quote this as 800 VAC - they are probably measuring under no load)
green to green/yellow pair 6.3 VAC @ 4 A, center tapped with green/yellow
yellow pair 5 VAC @ 2 A

-----------------------------------------
My amp calculated specs

GZ34 1.9A minimum filament winding

(2) 6L6GC, 1.8A. 155 mA
(3) 12AX7, 0.9A
Total 6V winding minimum of 2.7A, let's say at least 3 amps

[Helmholtz]

The only difference I see is V1, and the 6L6's being cathode biased on one version.

Funny as both schematics show grid bias.

[g1]

Regarding that PT on Reverb. I'm wondering about the 5V winding 2A limit being right at the GZ34 1.9A.

The spec should be for continuous use, it should have no problem delivering 2A on that winding.

[Helmholtz]

Sorry but the PT on Reverb is rated at 60mA (HT).
Also description says for 2x6V6 or single ended 6L6.
Looking at the footprint this is a tiny PT, even smaller than that of my 18W Super Champ.

2x 6L6 typically need at least 200mA HT current capability.
Even at idle AC current demand is around 80mA.
An undersized PT will overheat.

[misterc57]

Glad I have all your guidance on choosing a PT. Education and search continues.

Thank you

Here are some reference pics from Kim's amp.


.