New amp project using scrounged xfmrs, transformer questions...

[AmberB]

Hello all!  I'm working on a plan for a small amplifier using the transformers removed from a tube powered console stereo.  The stereo used a pair of single ended 7868s for the output tubes.  According to the RCA tube manual, I should be able to get about 11 watts from a single ended 7868 with 300 volts on the plate and a 3K output xfmr. 
My questions start with the power xfmr, when I test the B+ using 120 volts from my isolation transformer, I get a reading of 137 volts output.  That seems really odd to me and makes me wonder if a voltage doubler was used in the old stereo.  I've had the xfmrs for several years, and I didn't write down what make and model they came from, so I can only guess at that.
The amplifier used solid state rectifiers and there's no center tap for the B+ winding.

Second question, upon testing the output xfmrs, I calculate approx 5K ohms at 8 ohms using the voltage measurement method.  The turns ratio seems to be about 24.5 to 1.  That seems high for a 7868.  If you figure it for a 4 ohm load, 2500 ohms seems a bit low.
There's not a lot of data available for the 7868 in single ended use other than the one example in the RCA manual.
I'm planning on using 7591s instead of 7868s, but the specs are the same for both tubes.

It seems odd that the manufacturer would use less than 200 volts B+ on the plates of the 7868s, which is why I wonder about the possibility of a voltage doubler in the power supply.  I'm thinking that's the best way for me to go for this amp's power supply.
Then there's the question of load matching for the output transformers.  Is 5K too high?  Is 2.5K too low?

The idea is to make a small 2 channel guitar amp with separate channel inputs so that I can plug stereo effects units into the amp and get the stereo sound without using 2 different amps.

[AmberB]

Another thing I should mention, I did write down all of the tubes that were in the console stereo so that I could calculate the amperage draw on the heater winding.  There was 3.5 watts of heater current from all the tubes in the amp, so I have enough heater current for 2 complete preamps.  I can guess at the B+ power load capability based on what the various tubes were rated at on their specs, it should be more than enough for a couple of preamp tubes as well as the output tubes.  There were 12 tubes in the original console stereo, including all the radio tubes.

[DummyLoad]

More than likely was a Delon doubler for that power supply. Use PSU Designer to simulate, take DC resistance measurements of the windings to improve accuracy.


--Pete

[PRR]

138V through a doubler makes 380V.

You can adjust voltage and current over a wide range but Power has to stay near the same. 300V on 3k is like 380V on 5k.

If you kept the PT, OTs, and rectifier type, together, it WILL work like before.

[tubeswell]

transformers removed from a tube powered console stereo.  The stereo used a pair of single ended 7868s for the output tubes…

upon testing the output xfmrs, I calculate approx 5K ohms at 8 ohms using the voltage measurement method.  The turns ratio seems to be about 24.5 to 1.  That seems high for a 7868.  If you figure it for a 4 ohm load, 2500 ohms seems a bit low.
There's not a lot of data available for the 7868 in single ended use other than the one example in the RCA manual.
I'm planning on using 7591s instead of 7868s, but the specs are the same for both tubes.

It seems odd that the manufacturer would use less than 200 volts B+ on the plates of the 7868s, which is why I wonder about the possibility of a voltage doubler in the power supply.  I'm thinking that's the best way for me to go for this amp's power supply.
Then there's the question of load matching for the output transformers.  Is 5K too high?  Is 2.5K too low?

The idea is to make a small 2 channel guitar amp with separate channel inputs so that I can plug stereo effects units into the amp and get the stereo sound without using 2 different amps.

So parallel SE 7868s into the same OT wants to see a Pr:Sec impedance ratio that is half of what if would be for a single 7868.


Don’t see a problem with what you’re going to do. Unless you’re going to only run one 7868 into that OT, in which case double the speaker load.

[kagliostro]

Don't know if on some other apparatus was an old standard, but on old stereo Telefunken, Groundig and other brand radio often the speaker impedance was 5.5ohm


Franco

[AmberB]

So parallel SE 7868s into the same OT wants to see a Pr:Sec impedance ratio that is half of what if would be for a single 7868.


Don’t see a problem with what you’re going to do. Unless you’re going to only run one 7868 into that OT, in which case double the speaker load.

I have 2 output transformers, this was a stereo receiver that had 1 output tube per channel.  My goal is to build a dual output guitar amp with a preamp for each output, 2 amps in 1 chassis so that I can plug stereo effects pedals into the amp and get stereo output.

[DummyLoad]

So parallel SE 7868s into the same OT wants to see a Pr:Sec impedance ratio that is half of what if would be for a single 7868.


Don’t see a problem with what you’re going to do. Unless you’re going to only run one 7868 into that OT, in which case double the speaker load.

I have 2 output transformers, this was a stereo receiver that had 1 output tube per channel.  My goal is to build a dual output guitar amp with a preamp for each output, 2 amps in 1 chassis so that I can plug stereo effects pedals into the amp and get stereo output.

It was a stereo console, so perhaps the combined speaker load for each channel was somewhere between 4 and 8 ohms. Unless you know exactly what that was, then just shoot from the hip, use a 4 or 8 ohm load, neither will produce catastrophic results, just not optimal.

--Pete