VHT 2/50/2 – Power Transformer Heater Winding Specifications

Power Amps 2/50/2
1

Hello Fryette Support,

I own a VHT/Fryette Two/Fifty/Two (2/50/2) stereo power amplifier.

Could you please confirm whether its power transformer has separate 6.3V heater windings for the output tubes and the 12AX7/12AU7 driver tubes?

If so, what is the rated current capacity (in amperes) of the heater winding supplying the 12AU7 tubes?

I would also like to know whether replacing the two 12AU7/ECC82 tubes with 12BH7A tubes (600mA heater current each, compared with 300mA for 12AU7) is electrically safe in this amplifier, considering both heater capacity and driver-stage operating conditions.

I understand that the standard tube complement specifies 12AU7, and I am seeking technical confirmation before considering any substitution.

Thank you for your assistance.

2

Hi @dim

First check if they will fit, I think 12BH7A are a longer tube than 12A**

Also, they draw 10-20x the current from the high-voltage rail than a 12AX7 and a bit more than an 12AU7. And yes, double the heater requirement.

It’s not the kind of information I have to hand, so if @DavidPhelge spots this, then maybe be can comment. If you wish to try it then it’s your amp and experiment, but I think this is not the kind that we generally support. But the hunch is that the power demands are dominated by the power tubes anyway.

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3

The filament supply is common to the preamp, driver and power tubes. Current capacity is specified at 7.2A continuous.

Changing the AUs to BHs will increase the rated filament current load by roughly 10%, or approximately the allowable tolerance of the transformer itself, so not a major concern.

Plate dissipation with the BHs will increase 25%. Again, not an egregious violation in terms of overall power supply load, however, in this particular application, you will lose a bit of gain and the plate voltage of the AX input/phase inverter will lose a bit of headroom and clarity. In addition, the NFB gain will decrease slightly making the Depth and Presence controls marginally less active. Probably not a deal breaker there either.

In general, the Two/Fifty/Two is pretty overbuilt as stereo tube power amps go, so you’ll not likely run into any serious consequences.

Are you just rolling, or is there a particular outcome you’re hoping to achieve? In any event, give us an update if you decide to give it a go.

Dave

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Hi Dave,

First of all, thank you very much for taking the time to provide such a detailed and informative reply. I really appreciate it!

I should mention that I’m using my Two/Fifty/Two as a stereo Hi-Fi power amplifier rather than in a guitar rig. It is currently running four 6L6 power tubes, and the 12BH7A tubes I’m experimenting with are TAD Redbase.

My goal is to achieve a fuller, more natural and cohesive musical presentation, while preserving clarity, instrument separation and stereo imaging.

After comparing the 12BH7As with the original ECC82/12AU7 tubes, I’ve noticed several differences that I find very appealing.

The 12BH7As seem to give instruments more body and density across the entire frequency spectrum, along with a stronger sense of musical cohesion or “glue.” The soundstage feels slightly wider and deeper, while the center image remains firmly focused and individual instruments retain their positions.

One particularly interesting difference is that cymbals and other instruments seem to blend more naturally into the overall mix, without losing their definition or stereo placement.

I’ve also noticed slightly less audible hiss with the 12BH7As, equally in both channels, with only the power amplifier running and no preamp connected.

There is no perceptible difference in overall listening level between the two tube types.

As an additional observation, after approximately three hours of operation, the power transformer’s external temperature feels practically the same with either tube type.

When I switched back to the ECC82s, the overall presentation sounded noticeably thinner to my ears.

Your explanation of the electrical effects of this substitution was extremely helpful, especially regarding the filament load, plate dissipation and changes in gain and negative feedback.

Given that my amplifier uses four 6L6 power tubes, would your assessment of the substitution remain the same for this particular configuration?

Thank you again for your time and expertise. It’s greatly appreciated!