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
I'm glad I asked!!
How about this one
Hammond P-T169VS 169 Series Line Isolation Transformer
* 175 VA capacity, 1300 secondary current ma.
* Primary 115 VAC, 60 Hz.
* Secondary 115 VAC
How about this one
Hammond P-T169VS 169 Series Line Isolation Transformer
* 175 VA capacity, 1300 secondary current ma.
* Primary 115 VAC, 60 Hz.
* Secondary 115 VAC
The Hammond P-T169VS is rated at approx 1.5 amps so that is more than enough but at $38.00 vs. $12.00 from the surplus link I mentioned you can buy 3 for all your isolation needs... 🙂
Regards,
Sal Brisindi
Regards,
Sal Brisindi
tubesandmore has a hammond thats only 35va and perfect for your amp. Costs 20 bucks. I am building a Harmony h303A clone right now, and I ordered a 120vct filament transformer. Its a little big fro the amp thats for sure...
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!
Thanks for the tips! I ended up purchasing a Triad N-68x for a 12AV6/50C5/35W4 project but it runs quite hot. I had intended to install another in a 2 x 50C5 stereo but I'm guessing I'll need one rated a bit higher....
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!
I usually use separate heater and B+ transformers.
6VAC transformers are very cheap.
And isolation transformers can be used for B+.
My current project is a 100 watt valve amplifier.
I use a 100VA 6VAC heater transformer and 200VA isolation transformer 230VAC to 230VAC.
6VAC transformers are very cheap.
And isolation transformers can be used for B+.
My current project is a 100 watt valve amplifier.
I use a 100VA 6VAC heater transformer and 200VA isolation transformer 230VAC to 230VAC.
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.
On my previous project I used 2 transformers back to back.
I reduced mains down to 2 off 6VAC windings, one for heaters and the other to drive a reversed 9VAC to mains transformer. This gave me 170VAC for B+.
I reduced mains down to 2 off 6VAC windings, one for heaters and the other to drive a reversed 9VAC to mains transformer. This gave me 170VAC for B+.
John,
The OP's amp contains 3 signal tubes, a 12AX7 and 2X 'C5s. The N-68X should be sufficient.
BTW, hot to the touch, but not so hot that you must immediately remove your finger, is OK. Obviously, decent ventilation is essential.
The OP's amp contains 3 signal tubes, a 12AX7 and 2X 'C5s. The N-68X should be sufficient.
BTW, hot to the touch, but not so hot that you must immediately remove your finger, is OK. Obviously, decent ventilation is essential.
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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