1960 Stromberg-Carlson Signet 33 Guitar conversion!

Natima

New Member
Here's the low down... I picked up this amp because I wanted a conversion project as a learning experience, and after watching a couple of YT videos and looking at the schematic of a few old PA amps it looked like a simple enough project to dip my toes into.

Below are the original schematic and an altered one with all my changes marked in red (Well... nearly).

A few things I am really thinking about, and I'm not really sure where to start, are as follows.
1. Converting to passive tone controls.
- Pros and cons?
- Complexity of the mod not worth it?
- Trying to find D-shaft pots to fit the original knobs in the values I need seems difficult.

2. Converting cathodyne to Long Tail PI.
- Much the same... is it worth it?

3. The one change not listed in my modified schematic is the replacement of the 30/30 C19A/B to a 50/50... which caused some arcing on start up of the 5U4GB due to exceeding the spec sheets capacitance values.
Now I have two options, reverse pins 2 & 8 and swap to a GZ34, or buy a 32/32 cap, and replace that.

4. Some small questions.
- Why no bypass cap in V2B cathode? What if I put a 25uf in there?
- Ways of making use of the (now disconnected) Phono pot?
- Mic1 and Mic 2 are identical... seems a little pointless having to identical inputs... is there a good way of putting V1B to better use and differentiating the two channels?
- Is V3A just driving the PI? Providing 0 actual gain?

https://photos.google.com/share/AF1...?key=aHJPS0dRMk9PWWhIalRPSDVNU0tManV2azgxZ19n

IMG_20190111_183752.jpg Nat schematic.jpg 49938659_606897876416288_3008680073235529728_n.jpg IMG_20190202_181658.jpg
 
1) they already are passive, just arranged with a tube between the bass and treble sections instead of after both.

2) difficult in a practical sense. You'd need another triode to make that work. I doubt you'd get much performance change out of it honestly. Re-working the driver also means fiddling with the feedback to make it behave.

3) shouldn't matter honestly. There is a large resistor between the output of the rectifier and the first cap, so the value of the first filter is not terribly critical. It shouldn't have any issues dealing with a 50uf.

4a) it adds some local feedback and reduces gain to not have a cap there. Unless you need more gain, and trust me these have plenty (I own a pair), you don't need to do this.
4b) its a line level input. Can use that for whatever line level source you like I suppose.
4c) it made sense in stock form, with a guitar I suppose you could alter the loading or add some filtering or something. Not a guitar player, no clue what would make sense
4d) V3A provides a significant amount of gain. Thats the voltage amp for the power amp section. V3B is the inverter, which is slightly less than unity gain.
 
5U4 may be weak, try a new one (measure B+ with old & new - that'll prove it). 50 uF should be no problem at only a little over 100 mA. Seems like there is plenty of gain without adding the cathode bypasses, but try it and see. If you don't like the range of the tone controls, you can try other cap values - keep them in the 10:1 ratio. Fender or Marshall style tone controls have lower loss but less adjustment range. They typically give a lower midrange dip in the frequency response, sounds better with most guitars than a flat response. Disconnect the feedback (remove R35) for more gain. If you don't like the sound, put it back. For a guitar amp, it's a good idea to have screen resistors - 470 to 1K is typical - in series with pin 4 for each 6L6. You can make one input "bright" by using a small value cathode bypass - like .01 - .1 uF. V3A-B can be changed to a 12AT7 long-tail pair, but it will have much less gain than the pentode used now. Main advantage of LTP is in overload - it clips symmetrically. Cathodyne can give nasty sound in overload, especially if feedback is used.
 
The tone controls provide a cut and a boost... surely they are active?

I think I'm just going to stick a larger resistor in input two for a cleaner first gain stage.
Any way of bridging the two inputs?

I'll try a new rectifier tube. I have a few lying around.

Gadget, I removed the phono input jack and the pot is currently totally disconnected. So I have a 500k pot not connected to anything that I am looking to make use of!

Would it be beneficial to replace R18 with a 1M to assist in taming blocking distortion?

I have been told I can implement a pre PI master volume by inserting a 1M pot after C12 and into the PI Pin 9. What happens to R18 in this scenario and do I need to add any capacitance to retain bias?
 
Its not really a boost so much as it stops cutting so much. Set flat there is a fair amount of loss through the circuit, but its basically even across the frequency range. To "boost" it, it simply cuts less at certain frequencies.
 
I'm having an interesting dilemma...
The 6U8 on one of the amps is causing issues.
Specifically, the amps is hummy, and crackly, and noisy unless I press down hard on the top of the 6u8, at which point it become crystal clear, and silent (no hum or crackle).
I've cleaned the socket with deoxit, and checked ALL internal connections which are totally solid with no shorts.

Am I really going to have to replace the tube socket? :C
Anyone have any tricks for an issue like this?
 
If the sockets are those thin brown wafers, then replacing them is a good move,,, specially if you have the amps stacked on the speaker,,, but those wafer sockets are troublesome....
 
Yes, you can add the master volume after C12, but you'll need a cap after it to keep the phase inverter bias - .01 or higher. R18 value makes little difference - leave it..
 
First of all, thanks to everyone for replying... this is a learning experience for me and I'm greatly enjoying it (While definitely trying to keep as safe as possible! All the help is much appreciated.
Here's another issue that appears to be happening with both amplifiers. Anyone have a clue as to what is causing this?
 
are all of the filter caps replaced? Usually thats caused by poor power supply decoupling, aka bad filter caps. The hum tends to support that theory.
 
I've been looking into good replacements for the main B+ line... I have switched out C19A & B in both though and that cured a LOT of 120 cycle hum.
It seems it's just as expensive to buy individual electrolytics as it is to buy a CE Dist/Amplified Parts multi-section can caps. The main thing I am looking into is the best configuration. CE/AP stocks 50/50/50/50 caps, but I am a little concerned about "tightening" the bass up too much. The stock are 40/30/15/50. And that last 50uf is tied directly to the cathode of the power tubes and doesn't need to be as high voltage as the rest.
I then thought about going 30/30/30 and adding a separate 50uf electrolytic for the power tubes.

This is the video of the phase interter issue.
The tube socket itself is a nice molded plastic or bakelite socket which by all means seems to be sturdy. It's also riveted to the chassis... meaning I'd have to mount it with screws I guess if I replaced it.
 
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with that big resistor off the rectifier I don't think you've got much to worry about in terms of stiffening the supply too much. The screen supply has a lot of sag in these. My plans for mine was to make them into proper hifi integrated amps, and fixing that voltage sag is one of the things I need to do.

I'd expect thats actually the root of the motorboating issue. The driver section is fed from the same branch as the screen supply, so output swings that cause the screens to draw more current affects the whole front end of the amp. 6L6 family tubes draw a pretty significant amount more screen current with output vs at idle.

might be able to re-tension the socket a bit if you carefully tweak the contacts inward.
 
Here's the low down... I picked up this amp because I wanted a conversion project as a learning experience, and after watching a couple of YT videos and looking at the schematic of a few old PA amps it looked like a simple enough project to dip my toes into.

Below are the original schematic and an altered one with all my changes marked in red (Well... nearly).

A few things I am really thinking about, and I'm not really sure where to start, are as follows.
1. Converting to passive tone controls.
- Pros and cons?
- Complexity of the mod not worth it?
- Trying to find D-shaft pots to fit the original knobs in the values I need seems difficult.

2. Converting cathodyne to Long Tail PI.
- Much the same... is it worth it?

3. The one change not listed in my modified schematic is the replacement of the 30/30 C19A/B to a 50/50... which caused some arcing on start up of the 5U4GB due to exceeding the spec sheets capacitance values.
Now I have two options, reverse pins 2 & 8 and swap to a GZ34, or buy a 32/32 cap, and replace that.

4. Some small questions.
- Why no bypass cap in V2B cathode? What if I put a 25uf in there?
- Ways of making use of the (now disconnected) Phono pot?
- Mic1 and Mic 2 are identical... seems a little pointless having to identical inputs... is there a good way of putting V1B to better use and differentiating the two channels?
- Is V3A just driving the PI? Providing 0 actual gain?

https://photos.google.com/share/AF1...?key=aHJPS0dRMk9PWWhIalRPSDVNU0tManV2azgxZ19n

View attachment 1407508 View attachment 1407509 View attachment 1407510 View attachment 1407511

That's a nice looking Tokai. Is it early 80's? What model?

I've tried doing conversions like this but have found them to be more tedious than they are worth for the tone you will get from them. It is easier to gut the chassis (and existing circuit) save for the transformers and tube sockets and find an existing scheme that uses a comparable power supply and tube complement. Quite a few Marshall circuits based on 6L6 as well as Fender Bassman, for example.
 
That's a nice looking Tokai. Is it early 80's? What model?

I've tried doing conversions like this but have found them to be more tedious than they are worth for the tone you will get from them. It is easier to gut the chassis (and existing circuit) save for the transformers and tube sockets and find an existing scheme that uses a comparable power supply and tube complement. Quite a few Marshall circuits based on 6L6 as well as Fender Bassman, for example.

It's a 1980 Tokai LS-100 that I am temporarily a custodian of for my cousin. I do have a '79 LS80 back home though :)
These have been a great project for me... really great learning experience and similar enough circuits to old guitar amps that a few tweaks here and there have gone a long way... although there are still obviously a couple of kinks to work out.
Rest assured I will post videos once I'm done with them! :)
 
Nice. I had a feeling it was of the golden era. The LS80's are also sublime instruments. I have a number of early 80's Tokai's myself. Two 100's and a 120. All solid flame tops from between '79 and 81.
Wi8uBs2.jpg


I look forward to hearing the amp after you're done tweaking and troubleshooting.
 
I've tried doing conversions like this but have found them to be more tedious than they are worth for the tone you will get from them. It is easier to gut the chassis (and existing circuit) save for the transformers and tube sockets and find an existing scheme that uses a comparable power supply and tube complement. Quite a few Marshall circuits based on 6L6 as well as Fender Bassman, for example.

After having about 8 conversions, PA to guitar, I agree.
Now I buy the PA based on the look of the chassis, cage/cover and the iron in it. Then gut it and build from a known schematic and tweak from there.
I don't even keep the original tube sockets anymore. It is time consuming to save them when gutting the amp. Then you have to get them good and clean re-tension them to prevent future issues.
 
After having about 8 conversions, PA to guitar, I agree.
Now I buy the PA based on the look of the chassis, cage/cover and the iron in it. Then gut it and build from a known schematic and tweak from there.
I don't even keep the original tube sockets anymore. It is time consuming to save them when gutting the amp. Then you have to get them good and clean re-tension them to prevent future issues.

Good point. I usually do the same, particularly with 9-pin sockets. I find octal sockets seem to be a little easier to salvage and tighten up.
 
Your PI problem could be a microphonic tube, but it does act more like a socket problem. Swapping tubes from one amp to another could point you in the right direction. Swapping in a single socket shouldn't be too bad, if you take pictures, and take your time. Just make sure you match the indexing of the tube socket , and make sure the B+ isn't lurking around.

I have done the PA to guitar conversion a few times with good results. My only real advice is, If you are keeping the original iron, then just modify the Preamp section, and leave the PI - Output sections alone. ( Feedback being the only exception )

Two identical inputs can be addressed by looking at the typical Low/High input circuit or even clean / Tremolo
Good luck and have fun *(But be safe)
 
So one of these amps I'm leaving pretty much stock but with new jacks, and replacing bad capacitors.
The one I've already modified somewhat is getting a full treatment, taking some inspiration from the marshall plexi preamp.
Below is an updated schematic... the only change not listed is all the B+ filter caps are being swapped out for 30uF.

Am I going to be causing any harm to the transformer by switching out the 470k 6L6 cathode bias resistors to 220k?
hkBcMGxMPVNuKowjrEpu8bFyCvU1OjAc9yyyFyH4ei_9YAEEeJ28ieAG1y4CSUm9pUjSVRRarWolOrI9PWFAHYDZE5CID-6SxbQZAFYkmFfoIH9z-19asnRQaWovPzigjCWA-nbG8wW_YAUmSBEaqfjqoCCBVQfWcnDycatotUzvwGsV9gbpyU9z-29ANNOMZ1HFCBv5nsq39bX1aTW0IwpFfTawZ7aaW0mkLppJBS2sPtgbBhV_eHvcVLvUOZSd7k6VfsxXNpE4hIZFSQnYPSTWgaOcY5Tnwfq7FIPVN1ZOzo7zrLd4Tq_nMMEmYLKlwCPxGrBvqZhK-Pk7_C_HHRhu1562iI3kxpJ8unLKXl_A1YU6oauDQ_jI_m9eDB9XGWKIVlKpOjddcpyR3qO2AM67GIjCFqwvm-TlTIJVPQjpOJ3uC5qST2G8DSqiTF0wfVuVZBCdAtk1G4lP5KaOe8a27FpN9vYuJnEdomtEIxk8xxAxbL-4eO_9p78ixKv3Pz6zpcb5u3X8q1-W1RJdMcYF2YS25knGArd4TuLNYlX0YmI_SDb0PmhkM3FiPOm4FjZELe_GjKaNEIzW6PPg0H5LQDDkNvCpLsdE8oZmt_MD4XaUA6T9KaiTFuwewCLQbA78TlILTQW7JicQD2OlcBKT6B2AoZI=w1813-h916-no
 
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