Switching It Up

printer2

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Just an idea I have percolating. Rather than multiple tweaks I wanted one switch to change the nature of the sound of an amp. Or two switches, we will see. The main alteration would be to change the phase inverter from a Long Tail Pair to a Paraphase inverter. I might put in a pot to dial in the balance (or imbalance of it) or find a resistor value I like and call it a day. Along with that the first stage is changed from cathode biased to grid leak. To top it off would be to change the output fixed bias to cathode biased to loosen up the sound and shave some voltage across the output stage. Also loose the negative feedback. It would be nice to switch in some sag in the power supply but the four poles of the toggle switch are used up. Again, subject to change but a possibility.

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Hopefully getting a more 5D3 vibe or tightening everything up for a Blackface response. Which brings up the
tone stack. The double pole switch can change from a scooped sound to more of a Bassman/Marshall response. The use of a three position, center off give three different flavors. Whether I go for an extra switch or just two positions with maybe the treble switch with a push-pull switch, we will see.

I need to finish an amp for someone else so this will be on the back burner for now. But it is something I thought might be presented for your enjoyment.
 

printer2

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And a complete left turn, I want to make a small stereo system to sit on my desk. A pair of 8" speakers, really small cabinets for them, we will see how it goes. The inspiration for this is a lightweight guitar I am building, hopefully about 2 1/2 lbs. I fumbled around trying to figure out if it was to be acoustic or electric and electric won out.

Cut out a roughly Tele shape.

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Cut out excessive weight, in the end I decided to eliminate the bridge pickup and will be making an aluminum bridge plate.

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Almost got the speaker cabinets done. I think I will stain the wood, the grill cloth is a weaved place mat from a Dollar Store. I did want to use a wider board but for some reason my mind went off to see the Queen and I cut up this one. Decided to just go ahead and try it, not like I have anything to lose. It did have me searching for thinner material for the front and back to give a little more space inside. Might have added a couple inches, the speaker probably will not notice. I think I am going to have the speaker up to the top and a vent on the bottom of the back.

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Have been thinking of the amplifier. I do have a pair of transformers from a stereo, the two tan looking ones that are joined together at the bottom right. But they need a little heftier transformer than a single 5W amp. The other funky tan one at the bottom is the mate to the transformers, A monstrosity from a European console stereo. But as much space as the two speakers are going to take already I don't know about a big horkin' chassis for the amp. Do I use these transformers? Do I find a more creative way of powering them? Should I split the power supply from the amplifier? Go Solid State or a preamp tube with a SS amplifier?

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printer2

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As much as I would like to have a switch to do the above conversion of a LTP amp to a simple Paraphase, I do not think I can fit it in the amp cabinet. But this (squint with the right light) semi-Bassman-JCM800 wannabe might. If anyone sees any slip ups whack me over the head with it.

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andrewRneumann

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B9469D32-FB32-47AE-8B30-E7909B1E930A.jpeg
Admittedly I’m weak on Zeners. It looks like this diode is designed to provide protection from grid cathode arcing, and also to provide bias voltage. Am I right? Is this a proven design from another amp?
 

printer2

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IME, to keep the 2203 level of attenuation post cold clipper, this needs to be 1M because it is now in parallel with the following 1M grid leak.

View attachment 903480

I was using the 2203 schematic, mind you Marshall mucked around enough with values there might be one with a 1M in that position.

Marshall_2203_jmp_master_volume_lead_100watt.gif


View attachment 903482 Admittedly I’m weak on Zeners. It looks like this diode is designed to provide protection from grid cathode arcing, and also to provide bias voltage. Am I right? Is this a proven design from another amp?

Maybe 5.1V is cutting it a little close. The follower should not be much different than the source feeding the grid, a five volt difference seems like a lot though. I'll look it up to update what I have rattling around in my head. I have been using Mosfets as followers so I might be a little off with a cathode follower. Well there you go, don't need to be a zener. Just used to them being used for Mosfets. Can be an ordinary 1N4007.

aLwAuBG6EqB-kgcqmmLw29uOixl-30eSlb6eRcvD-tIyo_lVD4VJ4CMESsalwbVdvTFSAgT6wmRhO2kw8nU


http://www.valvewizard.co.uk/dccf.html

I probably should have looked it up. I added the 100R and Mr Valve Wizard says 10k, so who am I to argue?

And finally, did you mean to leave these in? I see the dual gang PPIMV.

View attachment 903485

Yes, they are suppose to be there but they should have a jumper to the X point. I need to fix that. thanks. They are there in case the wiper on the pot goes open. Updated the schematic. Thank you for taking the trouble to look it over.


Oh, I see. With the 1M at the grid and the 470k that is there, well it does not equal 470k. Sometimes my mind needs time to catch up even when it is written in plain English right in front of me. Shoot, fix the schematic again.
 
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printer2

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andrewRneumann

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Yes, they are suppose to be there but they should have a jumper to the X point. I need to fix that. thanks. They are there in case the wiper on the pot goes open. Updated the schematic. Thank you for taking the trouble to look it over.

You're welcome. I like that fail-safe. I wish there was a way to do it without attenuating the signal driving the power tubes though. I don't have any great ideas off the top of my head. Maybe you could just make them a little bigger and assume if one tube stopped working, you wouldn't keep operating the amp long enough to get one of the tubes to run away on you. It's good food for thought though. The design I'm working on has dual-gang PPIMV too and I didn't think about this. I was planning on installing fuses in the cathode circuit to protect the tubes.

Edit: Oh never mind--I just looked up the grid leak resistance on a 6L6 and realized you planned correctly. You kinda did what I suggested. I just didn't realize the datasheet suggests 100K max.
 

printer2

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You're welcome. I like that fail-safe. I wish there was a way to do it without attenuating the signal driving the power tubes though. I don't have any great ideas off the top of my head. Maybe you could just make them a little bigger and assume if one tube stopped working, you wouldn't keep operating the amp long enough to get one of the tubes to run away on you. It's good food for thought though. The design I'm working on has dual-gang PPIMV too and I didn't think about this. I was planning on installing fuses in the cathode circuit to protect the tubes.

Edit: Oh never mind--I just looked up the grid leak resistance on a 6L6 and realized you planned correctly. You kinda did what I suggested. I just didn't realize the datasheet suggests 100K max.

I plan on using 12AB5's but same specs as the 6V6. Or a 15CW5 would be cool now that I think about it. The way I looked at it also was the PI has more to give running the smaller tubes than the 6L6 so a little loss going text book will not hurt. Many guitar amps bend the spec and then if biased 70% or above fry the tubes with the grids running away with high voltage and playing on 11. I bet that the 70% rule for fixed bias is in balance with the grid leak resistance. If the resistance is lower the electron flow past the grid could be higher, mind you I have no problem with 70%. But bend too many specs, the resistance, operating at high voltages, squeezing out the most power with lower impedance transformers. But I am guessing you get all that.
 

Lynxtrap

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I was using the 2203 schematic, mind you Marshall mucked around enough with values there might be one with a 1M in that position.

That resistor does not seem to exist in the Marshall schematic. What purpose does it serve?
Anyhow, with that and the following grid leak both being 1M you are very close to the 0.5 attenuation ratio of the Marshall.
 

printer2

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That resistor does not seem to exist in the Marshall schematic. What purpose does it serve?
Anyhow, with that and the following grid leak both being 1M you are very close to the 0.5 attenuation ratio of the Marshall.

No it doesn't. And my mind blanked out on it when Andrew mentioned it. It is close enough ratio with the 470 and 1M, but it does not hurt to get it right. It is there so that the triode always has a return path to ground on the grid when the switch is flipped. It may only be for a split second but it will cause a thump to be heard on the output. I can probably live with that but it is just good form to put it in. I am more concerned feeding the second stage that is switched out. I had a three pole switch to cut the signal to the triode but with a limited chassis area got rid of it. Will the signal leak across the switch to the 1M resistor? I hope not.
 
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printer2

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Did some thinking and managed to make a real close circuit that uses a four pole switch to go from a Plexi to a 2203 circuit. Then i realized I had the bright tone shaping network in the normal channel. I could not believe O could make a mistake like that, but it was looking me in the face. So I had to redo it and get it right, wasn't sure if I could do it. I think I managed it. The only difference is the treble peaking resistor capacitor pair is not before the 1M volume pot but after it with the 1M grid resistor taking the place of the volume control as far as the resistor and capacitor is concerned. The tone stack is just some added versatility.

SrCwbmd.png
 
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Lynxtrap

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Looks interesting, that's a lot of switching! :)
A couple of reflections just from a quick look:

They way the switching is shown in schematic, it seems you have two "treble peakers" (I assume you mean the 470pF/470k combo) in the 2204 mode? Always hard to tell how an amp will sound from looking at schematic, but the Hi input channel sure seems bright.

Engaging S1D will put the cathode resistors in parallel for a sum of about 1.1K bypassed with a 250uF cap. Is that intended?

Personally I think I would have split up the functions between several switches for more flexibility.
 

printer2

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Just two switches, a four pole and a two pole three position. The previous schematic has a single two pole to put in a gain stage but that does not make a Plexi. Yes there are two treble peakers in the 2203/4, a lot of bass does make for good distortion in higher gain amps. I am more not inclined to put in the treble bypass around the volume control but I put it in to be more accurate to the original circuit. That is the first thing I will probably lose. Also making it more bright is the cathode bypass and the 0.0022uF coupling cap.

Thank you for catching the resistor. Knew I should have checked on the parallel resistors, should be 820R so a 910R. Fixed it. The 1.2k goes with a 2.7k which is what I had for the channel one, forgot to change that as I was used to seeing the resistor pair. The purpose was to produce two iconic amps in one package with the flip of a switch. It is easy to add switches to change the circuit, hard to get rid of them.
 
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2L man

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S1c and S1d operation is not clear to me when cascaded triodes work opposite phase? Also isn't the S1D fifth contact on S1?
 




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