PT Help

[C Moore]

I need a PT for a PA head that I "converted" for guitar. It takes a 5U4-G rectifier. The specs on the schematic are:
385 VAC x 2, CT, 140 MA
5 VAC
6.3 VAC, CT

Can I use this Hammond:
274BX 198 115 60 375-0-375 175 5 V@3 A C.T. 6.3 V@6 A C.T. X19

Thank You

[paddy]

Looks like a suitable replacement to me.

Your B+ should be down by about 14 Volts or so,but i doubt that is
anything to worry about.
The filament winding is rated at 3A,which is the specified draw for
a 5U4-G.
At first glance it might seem that this would be running on the edge of the
envelope but Hammond build their transformers very well and I don't
think you would have any troubles at all.

cheers,
paddy

[Phil_S]

Edcor makes several that might be appropriate and pricing may be better than Hammond. http://www.edcorusa.com/Products/Browse ... eriesID=37

[Andy Le Blanc]

That looks close to a generic 50w fender drop in.

[C Moore]

Appreciate all the help so far. This is a 6L6 x 2, cathode bias amp. I am more use to looking at a fixed bias amp, so forgive my naiveté. If I am reading the schematic correctly, the 6.3 V supply has a center tap that is connected to the power tube bias supply. Is that common?
Thank You

[jjman]

Appreciate all the help so far. This is a 6L6 x 2, cathode bias amp. I am more use to looking at a fixed bias amp, so forgive my naiveté. If I am reading the schematic correctly, the 6.3 V supply has a center tap that is connected to the power tube bias supply. Is that common?
Thank You

It is a valid option to connect the CT of the 6.3V winding to the cathode of cathode-biased output tube(s.) It would not be valid to connect it to the "bias supply" of a fixed bias output tube since that would be a negative voltage.

There is no bias "supply" in a cathode biased tube, strictly speaking.

It’s also valid to connect that CT to ground or to not use a CT and use the artificial CT resistors to ground as on many Fender BF/SF amps.

[C Moore]

Appreciate all the help so far. This is a 6L6 x 2, cathode bias amp. I am more use to looking at a fixed bias amp, so forgive my naiveté. If I am reading the schematic correctly, the 6.3 V supply has a center tap that is connected to the power tube bias supply. Is that common?
Thank You

It is a valid option to connect the CT of the 6.3V winding to the cathode of cathode-biased output tube(s.) It would not be valid to connect it to the "bias supply" of a fixed bias output tube since that would be a negative voltage.

There is no bias "supply" in a cathode biased tube, strictly speaking.

It’s also valid to connect that CT to ground or to not use a CT and use the artificial CT resistors to ground as on many Fender BF/SF amps.

Thanks for the info. I understand your point about bias "supply" in a cathode biased circuit. Mistakes in terminology are typical of someone like me who is just starting to learn. I've got a long way to go.
Thanks Again

[C Moore]

Another question. Do the specs from the original PT secondary seem high, for a cat biased, 2 x 6L6, amp?
"385 VAC x 2"
Thank You

[David Root]

No, not thru a 5U4G/GB. If you used a 5AR4 on the Hammond that would be pretty close to using the specified 770VCT with a 5U4G/GB, but maybe on the upside a little. You would get about 450-470VDC maybe at idle, depending on the cathode bias power resistor value. Don't hold me to those numbers, I didn't run the calcs.

Biasing closer to Class A should lower those voltages. High AB in cathode bias works OK, although "strictly speaking" it's not classical practice with cathode bias at these voltages and not a good idea to some people.

Use decent 6L6GCs and you should be OK.

[David Root]

No, not thru a 5U4G/GB. If you used a 5AR4 on the Hammond that would be pretty close to using the specified 770VCT with a 5U4G/GB, but maybe on the upside a little. You would get about 450-470VDC maybe at idle, depending on the cathode bias power resistor value. Don't hold me to those numbers, I didn't run the calcs.

Biasing closer to Class A should lower those voltages. High AB in cathode bias works OK, although "strictly speaking" it's not classical practice with cathode bias at these voltages and not a good idea to some people.

Use decent 6L6GCs and you should be OK.