1960 Stromberg-Carlson Signet 33 Guitar conversion!

[solderjunkie]

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?

Firstly, the attached scheme is not showing.

Secondly, those are not cathode bias resistors, they are grid resistors and that change in value will not affect anything as 220K is a typical value and reference the grid to ground.

If your intent is to bias the tubes "hotter" then you'll need to lower the value of the 250 ohm R24 cathode bias resistor. I'm guessing the original circuit is biased on the cool side but It would be prudent to calculate plate dissipation after changing the resistor. If you bias the tubes too hot you risk shortening their life or worse, red plating them, where too much current is flowing across the plates which can definitely damage the output transformer. You can safely cathode bias a 6L6 tube as high as 100% max plate dissipation or 19 watts. Generally speaking the best sound in a guitar amp is found between 70 and 100% of max plate dissipation.

Hope that helps.

 

[Natima]

Oh that totally makes more sense now! I was a bit confused by the way it was displayed on the schematic...
Here's a link to the schematic: https://photos.app.goo.gl/Rn25ugJJ2vvGsw1z5
From what I can tell pretty much the whole circuit was originally "cool" presumably for actual PA purposes... clean = better.
I can buy a couple different values, but what's a good starting point? 200 Ohm? 150 Ohm?

I guess now I have to ask the question, if I put 6L6GC vs 6L6GB in there would I be looking at different max dissipation? There's SO much mixed info out there on current production GC's.

This gets a little interesting. The two amplifiers have different components and schematics. I have uploaded a 2nd schematic to the OP album:
The one that CAME WITH GC's actually has GB/7027's listed on the schematic and C12 is listed as .033. (Though the tubes may have been changed as one pair are branded Phillips ECG, and the other Stromberg-Carlson).
The other one, has GC/7027's listed on the schematic and C12 as 0.1.
Both amps came with C12 as .022 or .02.
https://photos.app.goo.gl/QLQ3ETQuyHiLJQBYA

I cant find any other differences.

Side notes:
The one with the faulty socket is being kept pretty much stock except for the .022 coupling caps C14 & C15 (As opposed to .047).

Does anyone know where I can get a 1meg audio taper D-shaft pot? All the ones on eBay are extremely expensive old Centralabs.

 

[solderjunkie]

Oh that totally makes more sense now! I was a bit confused by the way it was displayed on the schematic...
Here's a link to the schematic: https://photos.app.goo.gl/Rn25ugJJ2vvGsw1z5
From what I can tell pretty much the whole circuit was originally "cool" presumably for actual PA purposes... clean = better.
I can buy a couple different values, but what's a good starting point? 200 Ohm? 150 Ohm?

I guess now I have to ask the question, if I put 6L6GC vs 6L6GB in there would I be looking at different max dissipation? There's SO much mixed info out there on current production GC's.

This gets a little interesting. The two amplifiers have different components and schematics. I have uploaded a 2nd schematic to the OP album:
The one that CAME WITH GC's actually has GB/7027's listed on the schematic and C12 is listed as .033. (Though the tubes may have been changed as one pair are branded Phillips ECG, and the other Stromberg-Carlson).
The other one, has GC/7027's listed on the schematic and C12 as 0.1.
Both amps came with C12 as .022 or .02.
https://photos.app.goo.gl/QLQ3ETQuyHiLJQBYA

I cant find any other differences.

Side notes:
The one with the faulty socket is being kept pretty much stock except for the .022 coupling caps C14 & C15 (As opposed to .047).

Does anyone know where I can get a 1meg audio taper D-shaft pot? All the ones on eBay are extremely expensive old Centralabs.

Yes, it is good to have a variety of values for the cathode resistor to experiment with. I like to have increments of 10 ohms. So dropping from 250....240, 230, etc... If you determine the plate dissipation with the existing circuit that will tell you how far to go out. For instance, if the dissipation is at 60% then yes, you might drop 50 ohms or so, but it is all variable depending on the circuit that you are building. You will need to just experiment.

I see now that the schematic specifies "6L6GB/7027" tubes. I would base your calculation for dissipation on whatever tubes are actually in the amp you are biasing. The specified plate dissipation is significantly higher for GC/7027 or GB than the standard 6L6. Remember, when you are measuring plate voltage when doing this calculation, measure the plate-to-cathode voltage, not plate to ground.

The variance of coupling cap values likely has to do with the voicing the original builder was trying to get. Different values affect the low or high frequencies of the signal. Perhaps there was too much low end in the original design, so they changed from a .1 to a .022.

This what you are looking for? https://www.tubedepot.com/products/alpha-24mm-audio-taper-pot-8mm-bushing-rv24a-10-15r1-da

 

[gadget73]

Is one of yours a Stromberg-Carlson and the other a Stromberg-Carlson, division of General Dynamics? Mine are that way. The older S-C calls for a 6L6GB, the GDI one wants the 6L6GC.

 

[Natima]

The two amps certianly sourced components from different manufacturers. One has CTS pots and the other has what looks like stackpole. The one with stackpole pots has General Dynamics branded caps, and the one with CTS has something else, I can't remember what off the top of my head.

Thanks for the link Solderjunkie... those are just what I was looking for. I ordered a couple and I'll stick a master volume in the shotgunned amp and see how it pans out!

 

[trobbins]

I find it better to basically get the amp working properly for starters, so you know what is working and if it has issues like poor sockets, or excessive hum, and to confirm what the gain structures are. As you appear to be using this amp as a learning curve, then you will learn a lot more about amps doing this, than just gutting it for starters.

Measuring the actual gains of stages, and their clipping levels, allows a basic awareness of where gain or vol pots are of most use (for a guitar amp). That would mean also confirming what feedback dB was being applied. You may then realise you have too much gain (and can for example think about repurposing V1) for typical guitar use. But there are a few places where stage gain can easily change a lot as you progress, such as bypassing V2B cathode, or changing the tone stack style (as you can repurpose MIC1 or MIC2 pots or both), or reducing or removing output stage feedback (eg. to add a ganged vol pot between PI and output stage), or removing V3A screen bypass.

It is also better to start with a standard guitar input circuit - eg. 12AX7 triode with 1Meg grid leak, 10k grid stopper, and bypassed cathode.

At some stage, you can look at bandwidth roll-off of each stage. With coupling cap values, that may be to reduce low frequency content to avoid large unwanted excursions of a guitar speaker, or to alleviate the extent of blocking distortion when purposefully overdriving certain stages. That then leads to where it is appropriate to add substantial grid stoppers.

Some amps have too much hum, and may need a few judicious wiring changes to bring that down, and can sometimes point back to the use of a multi-cap can.

You can also learn how to make your amp a bit more bullet proof using modern add-in techniques that don't change the character of the amp, but do minimise collatoral damage if something does go wrong. The absolute easiest add-in is to put 1N4007's in series with each plate of the rectifier tube, and to add a current sense resistor for each 6L6 cathode.

 

[Natima]

Thanks Trobbins.
I've actually been working on the amp incrementally. The first thing I did was just the 3 prong cord and input/outputs *just* to get it running and see how it sounded. Later I was attempting to control the enormous bass by swapping bypass caps and the power amp coupling caps. After that I've been working on preamp resistors, and converted V1A & B to cathode bias.

The bass is still rather uncontrollable, so I am shotgunning the coupling caps with .022's as soon as the parts arrive, and I may introduce a parallel input switch after differentiating the two inputs, just to see how it sounds.
T
As it currently stands the amp is almost whisper quiet unless I crank treble AND bass, and then I get hum and some motorboating (Just in SOME control configurations). 68k grid stoppers are also arriving in the mail soon for both inputs.
30x3 uF filter caps are also coming, and 50uf for the power amps.

The 2nd amp is currently just a repair job mostly. I'm leaving it stock aside from 1/4" jack inputs and outputs, and I will be comparing the two. Bone stock vs marshall-ified. From there I hope to make some more nuanced judgement calls about what to do with the each amp. I may decide to gut all the pots and go to a marshall style TMB/VOL/MV configuration if I can find the right pots.

 

[trobbins]

Yeh for guitar you may want to raise the coupling cap corner frequencies to well above 50Hz, so perhaps even drop the coupling caps to 10nF (eg. two 22nF in series if that is what you've got). And even drop the cathode coupling caps to something like 50Hz corner (eg. 1.5kohm and 2.2uF)

You also seem to be now getting in to topics that would benefit from tools that can confirm frequency response. Eg. an oscillator and voltage meter with known wide bandwidth, or a scope, or even better a probe/soundcard/spectrum analyser (which is what I use as it almost instantaneously gives a spectrum plot with embedded oscillator).

You may also want to start adding some grid stoppers in to stages that are going to be easily over-driven during testing (such as V3A, V3B, V4, V5 inputs).

 

[Natima]

So I will have 68k grid stoppers in V1A and V1B, and R18 if I am not mistaken is the 470k grid stopper for V3B. What kind of values should I be looking at for V4 & V5?

Regarding bypass caps... does it make sense to have a low value for V1A/V1B, and larger vaues for later stages?
This seems like a useful tool!: https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/
It's making me realize my conservative adjustments from 50uf to 25uf are exactly that, massively conservative!
My assumption is that a smaller cap in V1A will allow through less low frequency, and whatever harmonics are generated will be amplified in later stages with a more wide open bypass cap.
Like, I wouldn't want .68's in every stage?

 

[trobbins]

Nowadays the first stage grid stopper is typically reduced to circa 10k to minimise noise, as there is little benefit in choosing the default 68k commonly found.

R18 is the grid leak for V3B. For such a cathodyne that is exposed to high signal level, it is not uncommon to use 100k to 470k grid stopper (between grid and any other circuitry) to alleviate overdrive unpleasantness.

The cathode bypass cap increases gain above its corner frequency. In the first stage it is nearly always used to help attenuate hum from heater-cathode leakage, as the input signal is at its lowest level. If you want to roll off bass around 50-100Hz, rather than down at 1-10Hz, then lowering the value of any cathode bypass cap is appropriate.

Harmonic distortion control is usually related to high frequency roll-off, as harmonics are inherently high in frequency and not typically managed by low-frequency roll off circuitry.

 

[Natima]

I'm mostly done with my mods for now, I'll be posting a video soon. Honestly it's sounding pretty fantastic.
I added grid stoppers to V1A & B, replaced the filter caps, ran through the coupling caps with .022's, swapped V1A & B plate resistors to 100k, 6L6 cathode bias to 200ohm etc etc.
I will be putting in a master volume, but I have a couple of questions.

If R18 is the grid leak, and my master is going between C12 and pin 9 of the PI, where do I put my grid stopper and .01-.05 cap? Presumably the first tap goes directly to C12, middle goes out to a .05 cap, and that cap goes directly onto pin 9? Or into a grid stopper?

How does one go about paralleling inputs? Could I simply wire a switch that joins the positive of both input jacks?

 

[Natima]

EDIT: Ignore this post, I fixed the issue.

I'm actually now having an issue whereby channel 2 is putting out maybe 1/4 the volume of channel 1. Even at Max volume.
Additionally even channel 1 seems to have an overall lower volume despite switching R24 for a 200 ohm. Surely I'd expect higher volume now?

 

[Natima]

So I'm finally beginning to wrap this project up.
The 2nd amp is going to be a whole other story.

Below is my current schematic, changes in red, and things I am curious about in blue.
https://photos.app.goo.gl/CRke37NV8Ebf6ooF6

Currently the amp is extremely trebly, and with a lot of honky midrange, and pleasant distortion is not really being attained in a way I like.
How much does the value of the pots affect sweep of the tone controls? Would I be better off with 1M and different capacitors in the tone circuitry?

Can someone explain the values of R: 8, 9, 14, 15, 17, 20, 21, and 35? I know that's a lot.

C3 is presumably a coupling capacitor? Why is it so significantly smaller than the rest?

C11 should tame some treble right? I currently have it removed as the cap is dead. What are some good values to play with here?

I really want to tame the middle and treble now, and generate some more distortion.

 

[gadget73]

R8 along with C4 sets the turnover frequency of the treble control. C4 is a lower impedance at high frequency than the 820k resistor, so that provides the treble boost when the control is rotated in that direction. R8 is what gets signal that is below what C4 passes through the amp. Larger value at C4 will move the treble boost frequency downward, making it smaller will push it upwards. Making the value of the pot lower without changing anything else will push the response frequency upward.

R9 is the grid resistor. It also is used as the voltage divider that makes the tone control work. If it didn't have a load to work into it wouldn't have any real adjustment. If you increase the value of this, it should limit how much the treble control adjusts things but it will likely affect overall frequency response.

R14 sets a minimum DC resistance for the grid of the 6u8. If you remove it, the grid goes straight to ground and you'd have no signal at the grid with the bass pot all the way down.

R15 is part of the voltage divider for the negative feedback loop, and its part of what sets the bias on the pentode of the 6u8 voltage amp. It has much more effect on feedback levels though. R35 is the other half of this network. Between those two and the voltage level at the 4 ohm tap on the output trafo you get the power amp feedback loop. Increasing the value of R35 reduces feedback and increases gain in the power amp. That might be what you want for a guitar amp. Off the top of my head I forget how much stock feedback level is, but I seem to remember somewhere in the 15db range being about right. If you really want to know, bypass R15 with an electrolytic cap, set the output to get say 3v rms at the output, then pull the cap. The voltage difference between with and without the cap can be used to calculate the level of negative feedback.

R17 is the screen voltage supply for the 6u8. These are large to let the screen basically set itself. The pentode sections are pretty variable, and a large value screen resistor helps to make a wide range of tubes work properly.
Related to that, you should put C13 back in. Its the screen decoupling cap. With that removed the output from the pentode won't be as even.

R20 is both a bias and a load resistor for the phase inverter. Between R20 and R19 you get the couple volts of bias difference between cathode and grid in order to bias the inverter. The voltage across R20 is what actually drives the grid of the power tube after it.
R20 and R21 need to be the same value in order for the grid drive to be equal but opposite. They also need to be of low enough value that enough current is drawn by the pentode to allow for sufficient voltage swing in order to drive the output stage.

C3 works into a very high impedance, so the value can be low and the roll-off point will still be sufficiently low as to not destroy the LF response

C11 is an HF stability cap. WHere that rolls off is beyond hearing range, but it will keep the amplifier from oscillating. This one really needs to be installed and working. The stock value is fine.

Make sure all of the component values in the tone controls are right otherwise the response is going to be screwed up. The pots need to be somewhere near design value too.

 

[Natima]

What's going to be an acceptable value range for R35?
I'm not really at the point that I want to be poking around inside a live amp yet. Perhaps in a future project.

 

[gadget73]

Add a 10k pot in series with it and dial it to where you like the sound. Just don't reduce it below what it is or you may have oscillation issues. It will get louder as you reduce the feedback, so you'll need to compensate with the volume control.

Other thought, if the issue is too much brightness, removing or lowering the value of C8 will help with that.

 

[Natima]

I'm pretty much wrapped up! I have one microphonic preamp tube I need to replace. I might fiddle with the preamp gain a little in the future, but it's sounding pretty killer. Apologies for the cell-phone mic clipping out! The room has some natural reverb too.

I'd like to thank you all for all your help! You've been an invaluable resource and I hope to learn and study more, and ask you all about 1000 more stupid questions in the future!

 

[Natima]

Here's my final schematic
https://photos.app.goo.gl/ip4fvHWtqXMUySE7A

 

[solderjunkie]

Dude, that sounds awesome!!!

 

[Natima]

Is C8 just a high-pass filter?
Whats the relationship between the pot value and the caps on either side of the bass and treble controls?
I've been doing a LOT of googling, but 99.9% of tone stack information out there is for classic Fender/Marshall TMB tone stacks, which are a totally different beast.