New build_7591 option

[Sparkout-67]

Hi All,

I am presently in the process of a new build which, after two years of procrastination, is still in its design & layout phase.

The basis of the build started as a rebuild of the clean channel of my old DSL401, 4x 12AX7, 4x EL84, using the same PT and OT.
PT: Dagnall TXMA-00080
OT: Dagnall TXOP-00012 (4.5K Primary)

The preamp is nothing out of the ordinary, V1 (2x gain stages), V2 (gain stage + cathode follower), V3 (LTP PI), with V4 (12AX7) driving an FX-loop, I am confident that will at least work subject to building and tweaking.

My first question is, rather than using 4 x EL84 for the power section, could I use 2 x 7591's (cathode bias) instead ?

Dagnall TXMA-00080 Specifications:
Primary: 117/234 VAC 50/60 Hz.
Secondary-1: 307V @ 215 mA
Secondary-2: 6.7V @ 3.5A
Secondary-3: 25V @75 mA

When the DSL401 was up and running, the PT outs out a little over 400DCV after rectification (has been measured at 403DVC / 410DCV).
Mains voltage here in QLD, Australia averages 244V.

I have the new designed preamp estimated at around 9mA draw.
From the research I have done, if I'm not mistaken, the current draw from 2 x 7591's will be less than the draw of 4 x EL84.

From valve spec sheets.
2 x 7591 max signal (combined values for two tubes):
450DCV supply
Plate current = 94mA
Screen current = 22mA
Total draw = 116mA

4 x EL84 (combined values for four tubes):
300DCV Supply
Plate current = 184mA
Screen Current = 44mA.
Total draw = 228mA

I was thinking of aiming for a rectified B+ supply voltage of 375V/385V, am I correct in assuming the PT should be ok for the demand of 2 x 7591 ?

My second question, or rather concern is, will the OT be ok or, is the impedance wrong ?
Understanding the impedance requirements of output transformers for power tubes is where I fall down.

Thanks in advance.

[Bieworm]

Concerning the PT: this should be a fine PT for the 7591's. The B+ when built, the bias level and type of rectifier will determine how much DCV the plates will see. But I expect no anomalities.

Concerning the OT: a pair of PP 7591's like to have an OT with 6k6 primary. A quad of EL84 usually have a 4k OT primary. When you use a 4k5 primary on the 7591's there would be a lower output than with a 6k6 primary, but the 3rd harmonic will be enhanced. It is a matter of preference. Maybe check some examples online/youtube and try to get an idea towards what tone you're biased to.

[Sparkout-67]

Thank you for the reply Bieworm,

I'll be using a bridge rectifier, 4 x UF4007.

Now that I'm aware the present OT should be ok to get the amp up and running without destroying the tubes (as long as I get everything else ok) I might start with that and see how it goes.

Would there be any benefit in adding additional plate resistance inline with the OT to try and account for the impedance deficit ?

[Bieworm]

Thank you for the reply Bieworm,

I'll be using a bridge rectifier, 4 x UF4007.

Now that I'm aware the present OT should be ok to get the amp up and running without destroying the tubes (as long as I get everything else ok) I might start with that and see how it goes.

Would there be any benefit in adding additional plate resistance inline with the OT to try and account for the impedance deficit ?

Don't worry about that. It's not really a deficit anyway. The primary impedance of the OT is a designer's choice. If you look at Trainwreck amps you'll see that there are different primary impedances used depending on the character Ken was aiming for. Some like it sweeter, some like it raunchier.

[Sparkout-67]

Great.
I was looking at a new Mercury 1650HA 40watt 6K6Ω OT as an option but it would mean a bit of playing around due to size difference, although a possibility if if I'm not happy with the 4K5Ω I have.
That 1650HA is bigger than the mains transformer (TXMA-00080) from the DSL401 !