I have a 60's tube amp that connects directly to the AC. I want to add an isolation transformer for user protection, but don't know how much current it would need.
The tube lineup is 12AX7, 12AV6 and a pair of 50C5 output tubes into a small (6 inch) speaker. I believe this would be about 5 watts. It has a solid state rectifier.
Specs for the 50C5 are here
http://frank.pocnet.net/sheets/127/5/50C5.pdf
The Hammond 262E6 looks like it might do.
18VA, sec. 120V, DC ma 87, Fil. (heater) 6.3V @ 1.2A
Is 87 mA plenty of current for this?
Thanks!
The tube lineup is 12AX7, 12AV6 and a pair of 50C5 output tubes into a small (6 inch) speaker. I believe this would be about 5 watts. It has a solid state rectifier.
Specs for the 50C5 are here
http://frank.pocnet.net/sheets/127/5/50C5.pdf
The Hammond 262E6 looks like it might do.
18VA, sec. 120V, DC ma 87, Fil. (heater) 6.3V @ 1.2A
Is 87 mA plenty of current for this?
Thanks!
No,
If you look at the tube specs, you will see the filament takes 150ma so the filament string takes 150ma not to mention what the rest of the circuit will pull when you are using the amp. I would say you would need at least a 1/2 amp (500ma) isolation transformer and that is even at the bear minimum. 1 amp would be better.
Here is a 1.2 amp isolation transformer for $12.00 that would be great for your application.
http://www.surpluscenter.com/item.asp?UID=2006011508145064&item=15-1150&catname=
Regards,
Sal Brisindi
If you look at the tube specs, you will see the filament takes 150ma so the filament string takes 150ma not to mention what the rest of the circuit will pull when you are using the amp. I would say you would need at least a 1/2 amp (500ma) isolation transformer and that is even at the bear minimum. 1 amp would be better.
Here is a 1.2 amp isolation transformer for $12.00 that would be great for your application.
http://www.surpluscenter.com/item.asp?UID=2006011508145064&item=15-1150&catname=
Regards,
Sal Brisindi
Any time I fix an old line operated guitar amp or even an AA5 radio, I add an isolation transformer. For most applications I use the Triad N-68X. It is rated at 50VA which works out to be 435 mA. Mouser has these for $11.20. This is enough to run your 50C5 amp.
I used one in an Electrolab guitar amp powered by a similar tube lineup (12AU6, 12AV6, 50C5, 35W4). I mounted the transformer in the bottom of the box, below the speaker. The line cord goes directly to the transformer, and the transformer goes to the amp. In this case the transformer is always powered up. This has not been an issue.
They have a 115 / 230 volt dual primary and a single 115 volt secondary. I have been known to hook them up backwards making a 115 to 230 volt transformer. With a SS bridge I get about 280 to 300 volts under load. Add a $7 filament transformer from Mouser and you have enough juice to run a stereo P-P amplifier using 6AQ5's or 6V6's or even EL84's for under $20.
I used one in an Electrolab guitar amp powered by a similar tube lineup (12AU6, 12AV6, 50C5, 35W4). I mounted the transformer in the bottom of the box, below the speaker. The line cord goes directly to the transformer, and the transformer goes to the amp. In this case the transformer is always powered up. This has not been an issue.
They have a 115 / 230 volt dual primary and a single 115 volt secondary. I have been known to hook them up backwards making a 115 to 230 volt transformer. With a SS bridge I get about 280 to 300 volts under load. Add a $7 filament transformer from Mouser and you have enough juice to run a stereo P-P amplifier using 6AQ5's or 6V6's or even EL84's for under $20.
Tubelab:
I have seen some guitar amplifiers that use a 120v transformer into a full wave voltage doubler, and the result is 280vdc under load, with enough current to run at least a SE el84/6v6 or the likes.
Right now my problem with the isolation transformer is getting the voltage back down to proper operating levels... Hooked up my amp last night and the plates started glowing! I was using FWB ss rectification after an apparent 132vac out of the x-former. Seemed to be getting 180vdc UNDER LOAD. I replaced the bridge with a single 1n4007 ( like a 35w4 would be) and under load I got 140vdc, which is good for a 50c5. Ya know with enough filtering I dont seem to pick out any audible hum from that amp, and I'm only using a 47uf cap on the output of the 1n4001!
I have seen some guitar amplifiers that use a 120v transformer into a full wave voltage doubler, and the result is 280vdc under load, with enough current to run at least a SE el84/6v6 or the likes.
Right now my problem with the isolation transformer is getting the voltage back down to proper operating levels... Hooked up my amp last night and the plates started glowing! I was using FWB ss rectification after an apparent 132vac out of the x-former. Seemed to be getting 180vdc UNDER LOAD. I replaced the bridge with a single 1n4007 ( like a 35w4 would be) and under load I got 140vdc, which is good for a 50c5. Ya know with enough filtering I dont seem to pick out any audible hum from that amp, and I'm only using a 47uf cap on the output of the 1n4001!
A N-68X is probably all you need for the 2X 50C5 amp, if the B+ is SS rectified. What is the tube complement?
Take a look at this thread, where reverse connecting a N-68X was discussed. Reverse connecting a N-68X allows the B+ rectification circuitry and the series heater string to be energized from different sources and that provides considerable benefits.
BTW, if the 1st amp you did employed 1/2 wave rectification, "standing" DC was present. "Standing" DC can make transformers noisy and run hot.
Take a look at this thread, where reverse connecting a N-68X was discussed. Reverse connecting a N-68X allows the B+ rectification circuitry and the series heater string to be energized from different sources and that provides considerable benefits.
BTW, if the 1st amp you did employed 1/2 wave rectification, "standing" DC was present. "Standing" DC can make transformers noisy and run hot.
Hey Eli, thanks so much for the info! I'm still fumbling my way around these things, to an extent. The little stereo I was going to convert is a cheap old Archer, the tubes are 12AX7, 2 x 50C5 (actually 35C5 but I already have the 50C5) and a 35W4. I don't yet have a schematic, but I'm guessing it's still half-wave?
And yes, that first amp does employ 1/2 wave rectification. But I thought that implementing this rectifier bridge:
Isolation transformer upgrade for old guitar amps
would solve that issue!
And yes, that first amp does employ 1/2 wave rectification. But I thought that implementing this rectifier bridge:
Isolation transformer upgrade for old guitar amps
would solve that issue!
The 35W4 is a 7 pin mini 1/2 wave rectifier. The 35W4 is removed and SS full wave bridge rectification employed. Follow my posts in the linked thread. You have a nice socket setup of 3X 7 pin mini and a 9 pin mini. A 0B2 gas discharge regulator goes in 1 of the 7 pin "holes" to regulate g2 B+. Pentode IM distortion comes down, when you stabilize g2 B+.
The setup shown on the guitar amp page solves the "standing" DC issue, but doesn't take advantage of SS rectification's tolerance of good sized filter capacitors. Also, 60 Hz. hum associated with the tube heaters becomes 120 Hz. buzz, when that configuration is employed
Do what's necessary to maximize the old, dangerous, AC/DC POS' performance, from both a sonic and a safety perspective. Doing that includes: reverse connecting a N-68X, boosting the single winding to compensate for step down, and biasing the isolated series heater string off B+. Full bass extension is probably not possible, due to insufficient O/P "iron" heft. However, we may be able to get below the lowest note a double bass can play.
It is a given that highly sensitive speakers are needed to do critical listening with the low amount of power (approx. 2 W.) a 50C5 yields.
Definitely post the OEM schematic, when it becomes available.
I understand, just giving a heads up,,, That trans is hot with nothing connected to it, a compromised design, in my opinion...
I thought he mentioned a 35W4 also, I may have misread, if he's going SS rectifier...
BTW, you can't hold a finger on mine very long,, couple seconds, maybe...
Regards,
John
OK,,, If you say so, I believe you,,,
It was discoloring the wood,,, I recovered it with a screen top,,,Still, I want to get rid of the last bit of hum,,, so, I think a separate heater trans is in the works...
Regards,
John
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