Fender 5C3 build with 6L6's

[Cliff Schecht]

I finished my 5C3 build and it definitely deserves its own thread. It was nice doing a build again using only what I have on hand and to my surprise it worked great from the first time I turned it on. I didn't have any specs on the power transformer but the amp I gutted to fit this in originally used 6V6's and was fairly beefy so I figured it was good enough. It doesn't even get warm to the touch so I think I made a good assumption here.

I ended up going with 6SL7's instead of 6SC7's for a few reasons, namely that I have many more 6SL7's and 6SN7's and only 2 6SC7's (thought I had more..) and used slightly smaller plate resistors @ 220k vs. 250k in the original. I also used a 2.7k cathode resistor on the 2nd stage. I liked the idea of slightly less gain so that I could get a usable clean sound out of this amp without having to muck with the amps volume knob. It cleans up nicely using just the guitars volume but gets a great sounding distortion when cranked. I see why people are so crazy about the Tweed Deluxes, they really do sound great and it has a surprising amount of sustain!

The B+ sits at around 310V and the 6L6's idle at around 54mA giving me a plate dissipation of about 16.5W. It's definitely loud just through my test speaker so I'm thinking I will leave it as is as far as bias is concerned.

The only problems as of now are the amount of hum I get and the turn-on voltage spiking to over 500V. I installed a 250 Ohm hum balancing pot on the 6.3V tap which doesn't make a piss of a difference. The amp is usable in its current form but I'd like to figure out how to cut down on the hum a bit. I'm thinking it may have to do with the used and abused unmatched 6L6's I was using in the output stage, I'm going to try some better tubes and see if this cuts down on the hum. Since the transformer doesn't have a CT on the 6.3V winding, I can't use the elevated DC trick to knock down the hum.. The 500V inrush peak voltage is only a problem because the can cap I'm using is only rated at 450V. I'm going to use a PTC from the rectifier to the can cap to hopefully cut down this voltage spike to a more reasonable level. I'm very happy about that can cap working out so well though, it's a NOS Sprague unit from 1952 that leaked less than 300uA on both sides (it's a dual 10uF, 450V unit) before I reformed it. The leakage went down a bit more after reforming which was quite surprising to me.

I did this build point to point but was careful to plan it out and not rush through anything. I must say I'm very happy with the results considering this was a completely free build . Now I just need to label everything and find it a proper home for practice/gigging. Soundclips will happen when they happen, time is not on my side these days..

Also notice the strange Vitamin Q cap I used on the input. It has one lead coming out the bottom and a threaded outside connection. I ended up just soldering the 68k input resistors directly to the top of it. Seems sturdy enough, at least for now.

Here are some pics:

[Cliff Schecht]

Tube swaps killed the hum problem. A set of Mullard 6L6's in the output and different 6SL7's in the preamp stages (short bottle GE and RCA instead of taller Sylvania's) really made the difference. The Sylvania's have slightly more gain though so I find that these work well in the PI with the RCA in the first stage. Balance pot on the 6.3V heater does work as it should also.

I can also get a great clean sound by simply switching to 6SN7's. I'll have to play around with using these mixed with the 6SL7's to see what is the most usable for actually playing with the band..

Also, I originally left all of the tranny wires long as is evident in the pic (the huge heatshrink bundle at the bottom is the 10% feedback windings). I think I'm going to clip them down, this amp is definitely staying in the arsenal and I need to make sure it's strong enough for practices/gigging.

[Chappy]

Cliff

Looks good. I love these old amps. Free builds to experiment with. I have a Bogen Challenger 30 from the 50's that I'm working on. It already sounds real nice. Can't wait until I get the time to turn it into a wide panel tweed style amp. I also have a Grommes little jewel which will become a 5e3 variant at some point.

Steve

[Cliff Schecht]

5E3 is cool but using all octal tubes is cooler .

[Andy Le Blanc]

A couple tweaks for the fixed paraphase...

A 500k pot. on the inverted stage plate, this does the DC balance.

And a 10k pot in place of the 6.8k inverted signal source to do the AC balance.

The balance really affects the presentation, efficiency and band width.

fun to noodle with.

And..... your one resistor and connection away from a floating paraphase.

Another whole flavor of dirt when you want it.

[Jana]

You can elevate the heaters. Just tie the common point of the resistors you are using to create the faux center tap to your elevated voltage.

[Phil_S]

...I originally left all of the tranny wires long as is evident in the pic (the huge heatshrink bundle at the bottom is the 10% feedback windings). I think I'm going to clip them down, this amp is definitely staying...

Cliff, if it isn't broken, don't fix it. You've solved the hum problem and you report the amp is working well. Sometimes fine tuning is counterproductive. Just my 2 cents.

Real nice build you did there. It shows the benefits of being patient. I recently did a PTP Gibson GA-20. With apologies, not meaning to hijack your tread: http://home.comcast.net/~psymonds/GA20.htm This, too, was a bone pile build, including the chassis. Working with the existing holes is always a challenge. As I recall, I added one hole, maybe two.

Congratulations on your good results.

Phil

[Cliff Schecht]

This amp sounded great at practice last night! Not as much cut as I'd like but this may have to do with the darker cab I'm using (helps tame those Tele highs). Plenty loud and a good amount of sustain at full-volume. A great little blues amp for sure!

[Cliff Schecht]

I've messed with this amp a bit since I last posted about it. The output transformer got switched out for a 10kCT Stancor A3311 and I'm only running 6V6's now. I don't think the PT I'm using is meant to handle anything over 6V6's and it's quite old (50's vintage) and so I don't want to risk melting it. I fixed a few other annoying things here and there (replaced the can cap with a 16uF 500V Atom for safety).

My only problem now is that there is still a good amount of hum from the heaters. I'm running it all on a balance pot right now (no DC elevation) but I'd really like to convert the preamp to DC heaters. I probably have a suitable power transformer to use but I've never actually done DC heaters before. Can anybody point me at some "standard" circuits? Any advice on implementing DC heaters in such a circuit?

[labb]

Cliff, I just recently built the Deluxe 5B3 which was the forerunner to the 5C3. Not a lot of difference in the two..There is a Pro Jazz/Blues player using the amp right now in his show..Just a great sounding amp for that type of music..Some numbers that you might like to compare..I am running 340 VDC on the plates of the 6V6's. played with that a little. Found that it sounded best at the 340 volt..Running 162 and 165 VDC on the plate of the second pre tube. and 97 and 92 on the plates of the first pre tube..I am running 6SC7's for the preamp tubes..

[Andy Le Blanc]

You can get dc off a 5v and 6.3 windings. Where is it starting?
If its just the first gain stage you can add on a rectifier and filter caps right
there, I've seen that in more than one high gain amp.
A bridge will reduce the I DC x 0.62 so I'd double check the fil. winding rating
if you go for all the heaters.

[jjman]

I would 1st change the 1st tube to DC via a wall adapter or "lantern" battery, as a test. You don't want to bother with DC if that's not truly a problem area. Could always be tube, power supply, shielding, or ground loop related.

[M Fowler]

Ground the cathode of V1 to the input jack ground.

[Alexo]

Whenever I've used a 6SL7 in V1, I've had to use DC heaters for it. Man., I spent many a long night trying to chase that buzz!

Definitely run them off a lantern battery first to verify that the noise is heater-generated.

Then if it is (and it is, at least in part), you can run a bridge rectifier off the filament leads.

It sounds counter-intuitive, but if you bridge rectify the 6.3 VAC winding with 1N4007's or UF4007's, you will get ...6.3 volts DC (more or less)! The normally negligible voltage drop across the diodes seems to bleed off just enough to bring the rectified voltage down to where you want it.

Then follow it with a big E-cap, like a 4700UF/35V between the + and - outputs and you are golden.

You can NOT use a heater center-tap with this arrangement! But you can use 100 ohm resistors to ground, or a balance pot.

I would not use this arrangement on more than 2 preamp tubes - but usually that is all that is needed, and you can run the power tubes off of AC.

Those old octals did not have the humbucking filament architecture you see in 12A_7's, etc..

Cool tone, but they sure are finnicky!

[Cliff Schecht]

I'm really contemplating adding in another transformer internally and just using a 7805/2x1N4148 to get ~6.2V. The phase inverter and power stage heaters are wired together and will get DC elevated CT's. I know the noise is heater related and I don't have the money to buy a lantern battery right now. Just tapping off of the 6.3V and rectifying can cause problems in itself. It adds a sort of sawtooth component to the signal that's hard to get rid of and the large capacitance after the rectifier actually throws transients into the other windings every time that cap has to charge (i.e. every time you hit a note). Regulators kill both of these problems and give you a guaranteed 6.2V.

I need to add in some resistors on the other heaters to drop them a bit as well, they sit at about 7.2V because the wall voltage is too high for a 117V transformer. It's always the same problems with vintage iron, luckily I have tons of power resistors .