DeArmond (Rowe) Guitar Amplifier Model R25T Restoration

[misterc57]

Considering this PT but there are no specs. So probably not.

https://reverb.com/item/78322438-gen...p-project-1956

[Helmholtz]

Meanwhile you know PT voltage and current requirements.
Did you check the Hammond transformer website?

What are the original mounting hole distances?

[mozz]

Considering this PT but there are no specs. So probably not.

https://reverb.com/item/78322438-gen...p-project-1956

Ask on the antique radio forums, someone may know that model and be able to provide specs or Sams folder transformer cross reference.

[g1]

Considering this PT but there are no specs. So probably not.

https://reverb.com/item/78322438-gen...p-project-1956

Same seller had sold one with same part number, but different HV listed. For pair 6AQ5 power tubes:
https://reverb.com/item/52225286-gen...3-5-v-5y3-1954

[misterc57]

Okay so that one on Reverb is out of consideration.

Spacing is 64mm x 69mm center to center. I could possibly drill new holes to accept a different size.

The original PT seems on the small size looking at Kim's pics.

I did check a Hammond distributor site for PTs a while back and physical layout was an issue.

I am now looking at the Hammond site.

Going with these 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

Figuring 2 mA per 12AX7 so I need a HV handling of at least 161 mA?

Thank you

[misterc57]

Thoughts on this one? Physically may be a perfect fit. HV of 173mA too close to my requirement of 161mA?

[Helmholtz]

Meanwhile I think there's some evidence that this is actually a 25W amp:
1) It's model number.
2) The two 10" Jensen P10Qs.
3) The relatively small PT.

The only way to limit output power (and supply current) with given voltages is using a high primary impedance OT.
Typically two 6L6s are used with a primary impedance around 4.5K which would give something like 40W.
But with a primary impedance of 6.6k, this would reduce to 27W.
Likewise PT secondary current would be only around 140mArms.
So if I'm right about the OT impedance, the Hammond PT should be fine.

Two questions:
1) How did you come up with 155mA for the 6L6s?
2) Who's Kim?

[mozz]

Only problem I see is over amped on the filaments. You really don't want 7v where it's supposed to be 6.3v. Though lots of people do run them that hot I think it shortens the life. Have you looked at Edcor?

[Helmholtz]

No-load heater voltage is 6.95V. This will drop with load.
If still too high, adding two 0.1R resistors would lower by 0.6V.

[misterc57]

Meanwhile I think there's some evidence that this is actually a 25W amp:
1) It's model number.
2) The two 10" Jensen P10Qs.
3) The relatively small PT.

The only way to limit output power (and supply current) with given voltages is using a high primary impedance OT.
Typically two 6L6s are used with a primary impedance around 4.5K which would give something like 40W.
But with a primary impedance of 6.6k, this would reduce to 27W.
Likewise PT secondary current would be only around 140mArms.
So if I'm right about the OT impedance, the Hammond PT should be fine.

Two questions:
1) How did you come up with 155mA for the 6L6s?
2) Who's Kim?

At OT I applied 1.5 VAC on the secondary and measured 64 VAC on the primary. Giving a ratio of 42.6 : 1. . OT secondary tap is rated for 4 ohms. Do these measurements help to determine the primary impedance?

Kim is khmassey (on this board and thread) who sent me the schematics and has the same amp.

I'm now questioning the mA on the 6L6GC, should it be 210 mA?

[Helmholtz]

At OT I applied 1.5 VAC on the secondary and measured 64 VAC on the primary. Giving a ratio of 42.6 : 1. . OT secondary tap is rated for 4 ohms. Do these measurements help to determine the primary impedance?

A voltage ratio of 42.7 corresponds to an impedance ratio of 1820.
So primary impedance is 4 x 1820 = 7280 which seems to confirm my OT theory.

I'm now questioning the mA on the 6L6GC, should it be 210 mA?

It would be if Raa = 5600 Ohm and supply voltages wouldn't drop with output.
Remember I mentioned 200mA+ earlier.
Also PT secondary RMS current is at least 10% larger than DC current.

[misterc57]

Remember I mentioned 200mA+ earlier.
Also PT secondary RMS current is at least 10% larger than DC current.

Yes you did.

So where does this leave me on the max HV mA requirements? Seems this Hammond 376X will work?

Thank you!

[Helmholtz]

So where does this leave me on the max HV mA requirements? Seems this Hammond 376X will work?

See post #38.
The higher plate impedance, the lower current draw.
The Hammond 376X should work fine.

[misterc57]

I am leaning towards the Hammond 273BX which I think may be closer to the original PT. It does not have a bias tap but neither did the original. HV is slightly higher than the Hammond 376X at 700 VAC but lower than Kim's 760 VAC. Thoughts? Thank you

[Helmholtz]

Should work as well.