60's hybrid amp conversion to valves

BiBi · 10-11-2009, 04:55 PM

I thought I'd try again on this one since I didn't get a response in the vintage section!
I recently purchased an old Italian amplifier that uses 2 el84 output valves and a transistor front end. It seems likely it's from the late 60's or early 70's.
The power transformer has a marking of TA140MA on it which I'm going to assume means 140mA, at least I hope so.
My problem is determining whether or not the filament wires can handle running two extra valves in addition to the power valves. Is there a safe way of determining this without destroying the power transformer? Fusing perhaps?
Thanks for any help with this!

loudthud · 10-14-2009, 06:37 PM

Transformer ratings are based on temperature rise. Does the transformer run hot now? Can you hold your finger on the transformer for 60 seconds after the amp has been operating for an hour? If so, it is being operated below it's maximum ratings.

Measure the heater voltage with the EL84s at nominal line voltage. Two preamp tubes would be .6 amps or 10.5 ohms (6.3/.6). Add the additional load and see if the heater voltage drops below 6.3V. If not, you are probably OK.

olddawg · 10-14-2009, 07:05 PM

If the 6.3v supply is inadequate, couldn't you just use an additional cheap-small 6.3v filament transformer. It would be a lot cheaper than stressing the original PT and having to replace it.

BiBi · 10-14-2009, 07:36 PM

Cheers for that Loudthud! I thought there must be a sensible way to test it. I will hook it up tomorrow and see how it goes. Worst case I'll just add on a second transformer but there's little room as is.
If it means anything at all the filament wires are significantly larger than the 6v-4A transformer I already had around.
Thanks again for your help.

10-11-2009, 04:55 PM	#1
BiBi Junior Member Join Date: Sep 2009 Posts: 19	60's hybrid amp conversion to valves I thought I'd try again on this one since I didn't get a response in the vintage section! I recently purchased an old Italian amplifier that uses 2 el84 output valves and a transistor front end. It seems likely it's from the late 60's or early 70's. The power transformer has a marking of TA140MA on it which I'm going to assume means 140mA, at least I hope so. My problem is determining whether or not the filament wires can handle running two extra valves in addition to the power valves. Is there a safe way of determining this without destroying the power transformer? Fusing perhaps? Thanks for any help with this! Last edited by BiBi; 10-11-2009 at 08:40 PM. Reason: spelling

10-14-2009, 06:37 PM	#2
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 923	Transformer ratings are based on temperature rise. Does the transformer run hot now? Can you hold your finger on the transformer for 60 seconds after the amp has been operating for an hour? If so, it is being operated below it's maximum ratings. Measure the heater voltage with the EL84s at nominal line voltage. Two preamp tubes would be .6 amps or 10.5 ohms (6.3/.6). Add the additional load and see if the heater voltage drops below 6.3V. If not, you are probably OK. __________________ WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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10-14-2009, 07:05 PM	#3
olddawg Senior Member Join Date: Dec 2007 Posts: 498	If the 6.3v supply is inadequate, couldn't you just use an additional cheap-small 6.3v filament transformer. It would be a lot cheaper than stressing the original PT and having to replace it.

10-14-2009, 07:36 PM	#4
BiBi Junior Member Join Date: Sep 2009 Posts: 19	Cheers for that Loudthud! I thought there must be a sensible way to test it. I will hook it up tomorrow and see how it goes. Worst case I'll just add on a second transformer but there's little room as is. If it means anything at all the filament wires are significantly larger than the 6v-4A transformer I already had around. Thanks again for your help.

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