5c5 clone build

[old tube]

I'm about to order the parts for a 5C5 build. Why a 5c5? I want to try an amp with octal pre's and a paraphrase PI. Building it as a head.

I am going to go with the stock circuit as far as possible. Got my hands on some genuine unused Tung Sol 6SC7's. Plan to use Coke bottle 6L6G's and a 5U4G rectifier.

The Output transformer is rated as follows; Outputs: 4.5 volts @ 3-4 Amps + 360- 0- 360 HT + 2 x 6.3 volt outputs One at around 3-4 Amps and the other at 2 amps [ for the preamp section]. I can't find voltages for the 5C5 but I'm assuming that 360+ is going to be about right with 6L6G's.

Lastly the transformer set which is old, but tests good, comes with a choke which I could use or omit ... any views? and if it's a good idea what mods would I make to the mains filtering?

Is there any mods you'd recommend to the fender circuit? The death cap will not be present and I'll most likely put a standby switch in.

Many thanks

[David Root]

Good deal on those 6L6Gs! To protect them I'd put 470 ohm screen gridstoppers, 2W or better.

If your PT HT is indeed 360-0-360 on the secondary output, even with a 5U4G you should see well over 400V on the plates, up to 450V according to a quick simulation I did on PSUD2.

This could pop those 6L6Gs pretty quick, even if you bias the cathode current down. Risky. Even if it doesn't you'll have way more headroom than Leo figured on so it would sound different.

300-0-300 gets you closer to 360V plate. With 80mA idle cathode draw, screens thru a 10K resistor should run about 280V. This is likely where Leo ran it. No guarantee, he did over-run his tubes later on so I could be all wet here. But I don't think he would have designed for 450V plate.

[old tube]

Thanks for the reply. Hmmm ... perhaps the transformers are a bit over spec. I could certainly use them for something else that need that grunt. Apparently the Output transformer and choke were rated for 50 watts. Perhaps I should get appropriately spec'ed transformers for this build and keep the choke for another build that calls for it.

I thought that screen grid stoppers would be a good idea. I've read that they can mod the tone a little is this your experience?

[David Root]

On the screen gridstoppers, most people can't tell the difference at 1K and lower, but they need to be at least 470 ohms on 6L6s and 1K on EL34s to provide any real protection. If you go higher you will eventually notice some compression.

If it were me I'd definitely buy a good reproduction PT and OT and a 5W 10K sandbox resistor.

Mercury Magnetics Toneclones are good but rather expensive. They are closer than most would be to the originals.

Magnetic Components are good too and built old school style but may not be as close, as they seem to be designed for later narrow panel tweeds which would be higher voltage. OTOH they are a LOT less $$. Maybe look at a MM PT and an MC OT. There are other choices too.

Edit: I see you're in Europe, so MM & MC iron is going to get expensive by the time it gets to you. I would check the web for good Euro iron designed for MI amp use.

[jon]

The original PT was rated at 350-0-350 @ 140mA. Obviously, some PS sag was occurring. Part of the great TWEED sound is the power supply being overworked the under rated PT and the small filter caps. when you overspec components the amps generally sound stiff and lifeless, IMHO. I would look into having the correct iron made or go to Mercury. BTW the tweed Pro with 12AY7's in grid leak configuration is one of my favourite tweed amps, lots of compression and if you play clean on the edge of breakup it is heaven.

[overtone]

A grid-leak 5C5 is on my list too but I am still really unsure of the voltage output of the old 6516 PT. 350-0-350 seems too high?
From my notes and internet trawls the B+ for the plates is reported at around 365-385V. One UK builder went with the Hammond 302AX / 300V-0-300V and quoted B+ 360V at the plates which is where David Root's estimate above lands too. But that 302AX is rated at 198VA and I'm looking out for something less stout.
I would run with the Mercury if they would be more open with their data. It's like buying cats in a bag or someone saying trust me.

[JazzGuitarGimp]

I would run with the Mercury if they would be more open with their data. It's like buying cats in a bag or someone saying trust me.

I was very skeptical of this at first as well. Here are my experiences with Mercury (Patrick specifically):

If you ask him what a PT's HT winding voltage / current is, he'll tell you. If you ask him what an OT's primary impedance is, he'll tell you. However, what I have found to be the better way to get to the destination, is I tell him: I'm looking for a PT, OT, and C set for an amp that will have two KT88's in push pull, class AB1, with an HT of 400V, and a target output power of 80 watts. What do you recommend? Then ask him to send you the drawings for the PT and OT. If you need more information than what is on the drawings, shoot him an email and ask. He will answer your questions.

And I forgot to mention two things:

I also tell Patrick what rectifier I intend to use.
I cannot remember if they specify their PT winding voltages at no load or at full load, so you'll want to ask about that too.

HTH,
Lou

[David Root]

I think it's voltage at rated current load.

jon, where did you find that PT info on the 5C5? I cannot find that anywhere.

That Hammond 302 is one expensive PT. It's designed for 2A3 tubes, and you don't need the 2.5V heater windings either. There are cheaper 300-0-300 PTs around with closer to 150mA or so which would do the trick. I'm sure Hammond has one.

[old tube]

Thanks for the information guys.

Would this Hammond be suitable? Can get it in the UK for about $100.

[David Root]

That looks just right! Has a bit more 6.3V current than you need but that's OK. Much more reasonable price.

I almost forgot, if you have a 5R4-GY (not a 5AR4/GZ34) try it in place of the 5U4-G. See pic attached. I have two JAN RCA 5R4-G in my '57 tweed Twin and it sounds so much better with the 5R4Gs. Not even subtle.

In the UK you might find a Brimar 5R4-GY. They should be every bit as good as the RCA.

[overtone]

Lou you are quite right. I will ask Mercury directly, or get my dealer here to do it.
tony

[old tube]

That looks just right! Has a bit more 6.3V current than you need but that's OK. Much more reasonable price.

I almost forgot, if you have a 5R4-GY (not a 5AR4/GZ34) try it in place of the 5U4-G. See pic attached. I have two JAN RCA 5R4-G in my '57 tweed Twin and it sounds so much better with the 5R4Gs. Not even subtle.

In the UK you might find a Brimar 5R4-GY. They should be every bit as good as the RCA.

Many thanks for the confirmation on the transformer. I'll probably just go for that.

5R4-GY are quite expensive over here. Brimar's are not that common and command a good price. GEC's are outrageously expensive. Plenty unused brown base coke bottle RCA's available, about $50 for a boxed example. I've got a '57 twin reissue, which I recently re-capped with Jupiter caps, some CC resistors. Made a big difference to the quality of the sound on that amp .. as you said .. not even subtle! Kept all the original values, the new caps took away a "brittleness" in the tone and replaced it with what can only be described as tweed "smooth". Effect was immediate but got more pronounced over the first 10 hours of using the amp. Considering the new price of these from Fender, the quality of the components used on the board was shocking! (and quite noisy). It's a 2004 and the electrolytics were shot! NOS Sylvania 5U4G's in it now along with early '70's JAN Phillips 6L6WGB's, they sound fantastic in that amp (470V on the plates!). Got GE 6072A's in V1 and 2 and Mullard ECC83's in 3 and 4. The weber designed speakers are fantastic. I go through both channels on a switch bringing in the second channel for boost. Sounds fantastic with Fender single coils, P90's or Humbuckers. Bloody loud and punchy amp! I've tried pulling one of the rectifiers but it gets a bit too Neil Young too quickly for my taste.

First Picture is with the component upgrades. Second is what came out!

[David Root]

I got mine on fleabay from an outfit in S. Calif. IIRC they were $26 each, which I thought was fairly high at the time.


My '57 Twin was a scratch build with a real 1957 Triad OT, Fender No. 2818. I used all top quality stuff in it including a pair of early '60s P12Ns and early '60s Tung-Sol 5881s. It sounds great too.

I called it Corksniffer's 5E8A Lo Power tweed Twin or something similar; there's a thread on it here with pics if you're interested.

The last Fender RI amp I was in was a '65 Deluxe RI and what a POS. It had the 6L6 screen resistors on a PCB and they had overheated and burned up. They looked like 1W which is too small. I replaced them with 5W sandboxes. The build quality and structural design was very poor.

The last Fender I bought was a '62 Deluxe about 10 years ago. Sold it earlier this year, very nice amp.