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Will this work? Output transformer "repurpose"
#1
I've got a Bogen CHB-35A with fixed bias 7868 power tubes.

I'd like to reconfigure the PA to cathode bias. First, I'll drop the plate voltage down to 450V. Then I'll need to deal with the output impedance. According to the 7868 datasheet, a cathode biased pair of 7868s at 450v have a load resistance of 10,000.

https://frank.pocnet.net/sheets/168/7/7868.pdf

Here is what I have calculated for the Bogen's output transformer.

The output transformer measures as follows:
With 7.16vac applied across the primary, I get 0.184vac at the 4ohm tap.

7.16 / 0.184 = 38.8 turns. 38.8 * 38.8 * 4 = 6,039 primary impedance.

If I connect an 8 ohm speaker to the 4 ohm tap, the primary impedance would then be 38.8 * 38.8 * 8 = 12,128

To get a 10,000 impedance with this transformer, I would need a speaker load of 6.64 ohms. (38.8 * 38.8 * 6.64 = 10k)

And if all that is both correct and safe, can I then place a 39R 10W resistor in parallel with the 8 ohm speaker to get the 6.64 ohm load and the calculated 10,000 impedance?

I'm also calculating about 17% of the output being dropped across the paralleled 39R, less than 5 Watts

Is this math correct and will this work without frying anything?

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#2
The maths is OK, but if it was me I wouldn't bother with the 39 ohms parallel load. 12k instead of 10k is not a big mismatch, especially if you consider the much wider variation of speaker impedance with frequency.

On the other hand, the parallel load might give some protection if the amp is inadvertently operated without a speaker connected.
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#3
Quote:: To get a 10,000 impedance with this transformer, I would need a speaker load of 6.64 ohms. (38.8 * 38.8 * 6.64 = 10k)

If you want to get crazy with a speaker cab, you could run 2 16-Ohm speakers in series and a 8-Ohm in parallel to get the combined impedance to 6.4-Ohms which will also put you into the 10k ballpark @ 9.6k.
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#4
Quote:On 02/28/2019 @ 03:12, MAOpedals wrote :
According to the 7868 datasheet, a cathode biased pair of 7868s at 450v have a load resistance of 10,000.
I'd also comment that the PP impedance in such a datasheet is just the value they used to get the reported data for those conditions. The performance may be very similar with a noticeably different speaker impedance (ie. tap setting) that reflects back quite a different PP impedance value. The last graph shows the likely variation for pentode AB1 operation.

The cathode bias examples show performance levels that operate the valves at near class A level. I'd anticipate you would be wanting to push the output stage into clipping, which will not be the operating levels shown in the valve datasheet, and you may want to test what OPT tapping you connect your speaker to (4,8,16 ohm) to see what max power and acceptable sound you get.

You would also be wanting to add in some stoppers and check what feedback level is more aligned to use of overdrive.
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