Carolina Blues Special adaptation for an RCA 400 amplifier

[jmccanna]

Hi,

James here. I had the good fortune to find a beat-up RCA 400 projector at a garage sale.  It was missing a lot for a projector but had a full complement tube amp.  I have got the tube amp up and running as a guitar amp and it sounds terrific.

There is an extra tube socket for what was a 6v6 (used in the exciter circuit for pulling audio off of 16mm film).  I got this crazy idea that I could change the extra 6v6 to a 6sl7, rewire it as a dual-triode and implement a CBS style reverb.  It made sense to me, or so I thought, given that the CBS runs reverb off of 12AX7 (the 6sl7 is a 12AX7 precursor).  I have followed the schematic for the CBS as much as possible but am getting nothing.  I have attached the RCA schematic but it is not identical to my machine close but not entirely accurate.  There is a 6SL7 where the 6SN7 is on this schematic.

The problem is that I am not able to get the reverb working. I purchased a Fender reverb with the associated transformer from TubesandMore.com.  A MOD 4ABC31B and a PTF-22921 transformer.

I pulled the reverb signal off backside of the first coupling cap off the 6j7 plate and followed the CBS circuit from there - treating the 6SL7 as if it were a 12AX7.  I can get very faint reverby sound if I whack the reverb box but nothing else.  I may be barking up a tree but this would be cool to implement. The amp, as is, is creamy smooth with a little bit of bite. So, no real loss if it does not work but reverb on top would be awesome.  Thoughts?

Also odd is that I appear to be getting 16v at the heaters?

James

[sluckey]

Try this... Pull the signal from one side of R17 (100K) and return to the other side of R17 (V3 pin 4).

The dc on the filaments is normal. The center tap for the filament winding is connected to the cathodes of the output tubes.

[jmccanna]

Hi Sluckey,

You helped me (immensely) with my prior Hammond AO-63 conversion.  That was fun and it sounds great.  I will try your suggestion.  It had occurred to me while studying this just now that my first effort did not have a proper resistor between the in and the out. Your suggestion solves that problem.

Am I expecting too much to drive the reverb with 1/2 of 6SL7 and use the other half for the return?  It seems to have worked in the CBS.

James

[jmccanna]

Tried the R17 suggestion:  Since the schematic I have is a slightly different model (cannot find the one for my specific model) I discovered that there was no R17 on my unit.  I added one and it improved the sound but did not work for the reverb circuit.

I am about to abandon ship as it sounds great as is.  Sigh.

Thanks though for the help and the hope.

James

[jmccanna]

Hi,

On a whim and flight of fancy I pulled a short 3-spring reverb out of my old Ampeg solid-state amp and inserted it in the circuit.  Now, I have reverb. Not a lot but it is there.  I had ordered a Fender style tank and transformer thinking they would work.  A MOD 4ABC31B and a PTF-22921 transformer.  Maybe they have the wrong specs for this circuit.  I need to research this more. And, to figure out how to test the new reverb tank I bought.

At least I know I got the circuit connections right.  Suggestions on how to maximize the 1/2 drive from the 6SL7?

The only differences from the CBS schematic for my circuit is the cathode circuits on the 6SL7 each have a 2.2k resistor as I did not have a 1.5k on hand.

There is hope in the air again!

James

[jmccanna]

Hi,

Well, I inserted the extra reverb tank from my old amp and got some reverb.  I tried a couple of places to pull off the signal to the tank circuit.  One that works really well is right at the start off the plate of the 6J7 at the input. When I drive the reverb signal from there and return after the second pot for tone, I get real clean and full reverb.  I tried the same thing with the new tank and it did not work.

I am happy but a little perplexed - that however is normal.  If someone could point me in the right direction to figure this out I would go and study it.

Thanks all,

James

[tubenit]

Does the new reverb tank work at all with the old amp? 

with respect, Tubenit

[jmccanna]

Hello Tubenit,

First, thanks for sharing your circuit. When I studied it I thought that just might work. 

This little RCA projector amp is a powerful little sneaky beast!  Anyhow, I am going to check the new tank on another amp tomorrow.  I had the same thought that maybe the new one is dead but have already shut down my office/studio for the night.

Can I get back to you tomorrow?

The old tank that works is an Accutronics 8BB2C1B and the new one that does not is a MOD 4AB3C1B.

I have a Hammond AO-63 I converted with Sluckey's kind advice that I will test it on tomorrow.

James

[jmccanna]

Hello Tubenit,

While no one was looking I went back out and tested the new tank on the Hammond.  Nada, nothing, no sound.  Wouldn't have thought a brand-new tank would be bad.

Assuming I get a new on is there any issue with using this reverb tank with the CBS circuit?

Thanks.

[jmccanna]

Hi,

Success but embarrassingly so. I am revealing this only prevent others from being as boneheaded as me. I received authorization to return the reverb tank that was not working.   As I went to put it away I turned it over and realized I had not removed the foam inserts that were there to protect it during shipping.  So, when everything is right and confirmed right it is usually something really dumb.  I would say that this would qualify as really dumb.

The new tank works brilliantly.  I have tried several signal pull points and have settled on pulling the signal off the "in" to the tone potentiometer and the return to pin 4 of V3.  Sounds really good.  And, I can dial in the reverb precisely using the CBS circuit.

Thanks again to all and to Tubenit for this neat little circuit.

BTW, I needed a power transformer and a friend in the hobby sold me a chassis with a big transformer on it. The chassis turns out to be from a Bell and Howell projector and has nearly the same layout as this RCA. The difference is it uses 12AX7s.  I smell another little project.

Dumbly yours!

James

[Satchmoeddie]

A 6SN7 would drive the reverb much better than a 6SL7 would. The 6FQ7 is a 9 pin mini novar version of the 6SN7 and Ampeg used it as a reverb driver in quite a few amps. The pentode section of the 6DW7 would also work. That is an old Valco driver tube for reverb tanks.  It's a mini tube though.

[sluckey]

The pentode section of the 6DW7 would also work.

I can't find a 6DW7 in any of my tube manuals or even on the internet. Can you provide a datasheet?

[Tone Junkie]

Don’t feel bad. Every one of us have made a mistake like that. Real men own up. Like you did. Bill , happy playing