Channel Switching JCM800 Design? + BIAS pot question

[Colossal]

Excellent news, 'Doc,

"Lead dress", or the physical placement and routing of wires is an important consideration in tube amp building!

Noted! Time to invest in some zip ties, I think.

This is a topic that bears greater discussion. You will not want to bundle wires wholesale with zipties. In fact, it's the opposite. There is a way to plan and route wires specifically so that a certain minimum of space is maintained. There are some wires that should be close together, some that should only cross at ninety degree angles, if they must interact at all, etc. Moving leads around with a chopstick in an amp that is properly built but otherwise unruly and uncooperative is sometimes needed to make them stable. Again, separate topic and part of the art.

[ViperDoc]

Excellent news, 'Doc,

"Lead dress", or the physical placement and routing of wires is an important consideration in tube amp building!

Noted! Time to invest in some zip ties, I think.

This is a topic that bears greater discussion. You will not want to bundle wires wholesale with zipties. In fact, it's the opposite. There is a way to plan and route wires specifically so that a certain minimum of space is maintained. There are some wires that should be close together, some that should only cross at ninety degree angles, if they must interact at all, etc. Moving leads around with a chopstick in an amp that is properly built but otherwise unruly and uncooperative is sometimes needed to make them stable. Again, separate topic and part of the art.

Ok, great. So I went to Valve Wizard for some “beginner toe-dipping” and got so spun around, OMg. Where do you guys pick this up in the proper rations? I love math and the science behind all of it but it’s easy to get lost fast. Uncle Doug deals it out about as softly as I’ve seen, but I’d love a streamlined source if there is one. Any ideas? Thanks!

[Colossal]

Don't try to drink from the firehose. Take the concepts a bit at a time. Think them over, ask questions if you need clarity, and practice the concepts in your building. Some guys here have been doing this for decades. I am always learning. I will use the search tool here to look up keywords and then read, read, read. I have lost many hours of sleep reading into the wee hours, drawing my own schematics, tinkering on the bench. The Reading Material on Steroids sticky is a great resource. My copies of Blencowe and Keuhnel are hammered. I have accumulated tons of schematics, layouts, and PDFs on theory. Read all the early stuff from Crowhurst et al.

[ViperDoc]

Don't try to drink from the firehose. Take the concepts a bit at a time. Think them over, ask questions if you need clarity, and practice the concepts in your building. Some guys here have been doing this for decades. I am always learning. I will use the search tool here to look up keywords and then read, read, read. I have lost many hours of sleep reading into the wee hours, drawing my own schematics, tinkering on the bench. The Reading Material on Steroids sticky is a great resource. My copies of Blencowe and Keuhnel are hammered. I have accumulated tons of schematics, layouts, and PDFs on theory. Read all the early stuff from Crowhurst et al.

That’s about what it felt like. I loved the part where the author said something like “most believe that electrical current has a flow, but ACTUALLY,” and then I had to take a walk. Taking it slow is not my best skill. If I want to learn something, I MAKE one of them! Working out pretty good so far, all things considered, AND THANKS TO GUYS LIKE YOU, man.

[ViperDoc]

I've noticed that this latest 800-2204 amp has an insane amount of treble. Like the amounts of treble that were never intended for music. I'm using the same Avatar 212 loaded with Scumback H65/M65 speakers. Even humbuckers. I can get a usable tone if I dime the master and roll the treble ALL THE WAY OFF. Presence needs also be nilled. Do I have a bad filter cap? My power cable ground terminates together with the reservoir cap grounds. Should these be separated? What could be the deal?

[xtian]

I've noticed that this latest 800-2204 amp has an insane amount of treble. Like the amounts of treble that were never intended for music. I'm using the same Avatar 212 loaded with Scumback H65/M65 speakers. Even humbuckers. I can get a usable tone if I dime the master and roll the treble ALL THE WAY OFF. Presence needs also be nilled. Do I have a bad filter cap? My power cable ground terminates together with the reservoir cap grounds. Should these be separated? What could be the deal?

You built a proper 2204/2203, that's what! I think I mentioned this to you over a couple of threads: many players roll the treble and presence to zero. And for myself, I prefer to disconnect the treble peakers and bright cap.

[ViperDoc]

I've noticed that this latest 800-2204 amp has an insane amount of treble. Like the amounts of treble that were never intended for music. I'm using the same Avatar 212 loaded with Scumback H65/M65 speakers. Even humbuckers. I can get a usable tone if I dime the master and roll the treble ALL THE WAY OFF. Presence needs also be nilled. Do I have a bad filter cap? My power cable ground terminates together with the reservoir cap grounds. Should these be separated? What could be the deal?

You built a proper 2204/2203, that's what! I think I mentioned this to you over a couple of threads: many players roll the treble and presence to zero. And for myself, I prefer to disconnect the treble peakers and bright cap.

Well that makes me feel somewhat better, but the treble overload is a surprise. Looking around, it seems de rigueur to clip the bright cap on the input, change the values on the input, and/or clip the peakers. Are those the 470 K/500 pF pairs? I might have to play around with that. Any disappointment when bringing the treble-reduced 2204 back to full volume?

[ViperDoc]

Hey, I've been looking into 2204 treble reduction mods and I've found a few that interest me. The 2204 is designed to be run at full volume, seems like, so I'd like to introduce some switchable mods into my 2204 that will allow it to run in stock mode. Here's what I've got:

1) A switchable bright cap on the gain pot. I thought I would run an on-off-on switch to allow the 1 nF cap (stock)--no cap--and an alternative. Any recommendations on the value?
2) I see that Friedman runs a 500 pF cap in parallel to his V2A plate resistor, and several people have claimed it smooths out the distortion. Sounds cool to me. My diagram has the 100K plate resistor between the V2 pins 1 and 6, right over the socket. I assume I can do an spst switch on the chassis and switch the cap in. He also runs 500pF cathode bypass caps in several places. Search for the Smallbox 50 schematic. What does that do?
3) My diagram has a 10 pF cap across pins 1 and 3 of V1. The schematic has the 10pF cap heading into the wiper of the gain pot. Is this right?


How else would you tame the treble? Thanks!

[JD0x0]

How else would you tame the treble? Thanks!

Personally, I take the peaking networks and bright caps out. Those make the sound really toppy, IMO, but it's also part of that Marshall sound. If that's not enough I'd start snubbing highs off the cold clipper stage, with a cap. I like articulation, but I don't like fizziness or harshness. I think the stock ('High gain') Marshalls have a bit too many 'artifacts' riding on the upper harmonics personally.

[ViperDoc]

How else would you tame the treble? Thanks!

Personally, I take the peaking networks and bright caps out. Those make the sound really toppy, IMO, but it's also part of that Marshall sound. If that's not enough I'd start snubbing highs off the cold clipper stage, with a cap. I like articulation, but I don't like fizziness or harshness. I think the stock ('High gain') Marshalls have a bit too many 'artifacts' riding on the upper harmonics personally.

What exactly are the peaking networks? I’m not finding what that is. Thanks!

[pdf64]

Treble peaking filers are a type of shelf filter, such that the output has high frequencies at one level, lower frequencies at a lower level.
Treble peaking components here are R5//C4 and R10//C8 https://drtube.com/schematics/marshall/2204prem.gif
The shelf filter effect might be eliminated by shorting the resistor (all frequencies at top shelf level) or opening the cap (all frequencies at lower shelf level).

[martin manning]

1) A switchable bright cap on the gain pot. I thought I would run an on-off-on switch to allow the 1 nF cap (stock)--no cap--and an alternative. Any recommendations on the value?

It's up to you. I would probably go for a factor of two. You could also reduce the size, to say 250p/500p.

2) I see that Friedman runs a 500 pF cap in parallel to his V2A plate resistor, and several people have claimed it smooths out the distortion. Sounds cool to me. My diagram has the 100K plate resistor between the V2 pins 1 and 6, right over the socket. I assume I can do an spst switch on the chassis and switch the cap in.

It's a treble cut. There is high voltage there, so I'd rather pick a value and solder it in. Going plate to cathode will have the same result so if you really want a switch, placing a cap from the plate to the switch, and connecting the other side of the switch to the cathode would be safer. You'll probably need a large resistor across the switch to prevent pops too.

He also runs 500pF cathode bypass caps in several places. Search for the Smallbox 50 schematic. What does that do?

I would think very little at audio frequencies, but I don't have the schematic.

3) My diagram has a 10 pF cap across pins 1 and 3 of V1. The schematic has the 10pF cap heading into the wiper of the gain pot. Is this right?

10p is almost nothing there, as far as audio frequencies go, but it would help kill RF coming in from the guitar. Note it's 100p on the marshal schematic posted just above. Which schematic are you referring to where it's placed somewhere else?

[ViperDoc]

1) A switchable bright cap on the gain pot. I thought I would run an on-off-on switch to allow the 1 nF cap (stock)--no cap--and an alternative. Any recommendations on the value?

It's up to you. I would probably go for a factor of two. You could also reduce the size, to say 250p/500p.

2) I see that Friedman runs a 500 pF cap in parallel to his V2A plate resistor, and several people have claimed it smooths out the distortion. Sounds cool to me. My diagram has the 100K plate resistor between the V2 pins 1 and 6, right over the socket. I assume I can do an spst switch on the chassis and switch the cap in.

It's a treble cut. There is high voltage there, so I'd rather pick a value and solder it in. Going plate to cathode will have the same result so if you really want a switch, placing a cap from the plate to the switch, and connecting the other side of the switch to the cathode would be safer. You'll probably need a large resistor across the switch to prevent pops too.

He also runs 500pF cathode bypass caps in several places. Search for the Smallbox 50 schematic. What does that do?

I would think very little at audio frequencies, but I don't have the schematic.

3) My diagram has a 10 pF cap across pins 1 and 3 of V1. The schematic has the 10pF cap heading into the wiper of the gain pot. Is this right?

10p is almost nothing there, as far as audio frequencies go, but it would help kill RF coming in from the guitar. Note it's 100p on the marshal schematic posted just above. Which schematic are you referring to where it's placed somewhere else?

My mistake. The 10 pF V1A plate-to-cathode cap is nowhere to be found on the mojo schematic, but it is on the diagram. Thanks for the details on the switch wiring!

I see the 100 pF cap now in the Marshall schematic. I might have to switch that.

[ViperDoc]

Been looking at 1 MA Dual pots. Looks like Alpha or Bourns are about the only options I've found. Recommendations? Thanks.

[pompeiisneaks]

I buy most of my pots from antique electronics supply (tubesandmore.com)

Check this list:

https://www.tubesandmore.com/products/p ... ize%3DDual

has a bunch of different types of dual ganged pots. You can choose from many values, 100k, 1M etc, and A or B etc tapers.

~Phil