Mystery Amp - Take a look and see if you can help

[Chuck H]

Terminal strips come in real handy when laying out in a chassis like this. Radio Shack sells five lug strips (middle one shares the mounting bracket for ground) five to a bag for cheap. I like the old school toggle plates you have there. Before you snug it all in place... I've found that the easiest way to get one off face plates is to go to an engraver. You know, like for office supply. I just had one made for about US $25.00. I paid about US $60.00 + postage for something similar from an online service that specializes in amp faceplates. Not all these shops can do bigger than 12" so call ahead.

[Csquare4]

Terminal strips come in real handy when laying out in a chassis like this. Radio Shack sells five lug strips (middle one shares the mounting bracket for ground) five to a bag for cheap. I like the old school toggle plates you have there. Before you snug it all in place... I've found that the easiest way to get one off face plates is to go to an engraver. You know, like for office supply. I just had one made for about US $25.00. I paid about US $60.00 + postage for something similar from an online service that specializes in amp faceplates. Not all these shops can do bigger than 12" so call ahead.

Good idea on using the Engraver, I was really wondering what to do about the faceplate. I will need to find one in the area that can do there. Also on the terminal strips, I have a few in there already, I am going to lay it out on paper and see if I have enough attachment points, if not I will definitely be hitting the local Radio Shack. I had no idea they sold terminal strips.

[jmaf]

Nice job Csquare4, it looks great now, unrecognizable from the earlier version(for the better IMO).

8k primaries suggested a smaller tube, but Hammond probably wanted very low distortion with the higher Z primary. We should've guessed about the markings on the chassis...

[Csquare4]

Making progress...

For anyone interested in following the progress of my now "non" Mystery Amp, I have decided to go with a dual channel pre-amp (clean and "crunch") feeding a Fender "Super" 5F4 Tone Stack and PI, through a Lar-Mar PPI Master Volume, into a Cathode Biased 6L6 power section, which is highly derivative of the original Hammond design. The PT has a NFB with Presence control.

I have never had (or played through) a cathode biased 6L6 based amp, so I figured I would check it out then if I didn't really like the result, it would be easy enough to convert to a fixed bias. The full 5F4 TS required adding another gain stage, which makes this small chassis extremely tight! Yikes...with PTP wiring, I am spending quite a bit of time with layouts before soldering. My attempt at drawing the layout went out the window pretty quickly.

Anyway, here is a progress photo. I pretty much have the B+ circuit, the output section and the front-end complete. I need to finish the tone stack and master volume, then do some testing and tweaking. Hopefully I will have something complete in the next few days.

[jmaf]

Looking good so far!

2 cents worth: I'd add at least 470 ohm screen resistors to both pin 4's on the 6L6's

[Csquare4]

Good input...I was kind of debating on that, of course I don't have anything even close to 470 in a higher wattage resistor, which is way too typical. I should probably get some of those though.

[Csquare4]

For anyone still interested, I have completed 99% of the amplifier build. The chassis is extremely tight using 3-12AX7's, 2-6L6GC's and a 5U4GB Rectifier tube. I went with all point-to-point construction for the most part. There is very minimal low-level hum, which I suppose is to be expected in this kind of build, I will try to clean it up a little at a later date. The sound of the amp is very nice. It is very loud and crystal clear up until about 9'oclock on the volume, then it gradually just builds up it's roar. Very tweed sounding. It has plenty of low end, without getting farty and chimes to match. Cleans don't go too far up the dial, but that is ok, I have clean amps. I have never played a 6L6 cathode-biased amp before, I really like the way this thing responds to the guitar volume and pick attack. Frankly, I am pretty stoked.

I added a switchable "crunch" switch to the pre-amp, which is basically just switching both triodes of V1 into a parallel out configuration. This provides a little more warm meat to the output without going into death metal territory. With the crunch switch engaged, this thing really roars to the point of sounding very plexi-like. And loud. But manageable.

So here is a photo of the innards...I added the 470 ohm Screen Grid Resistors. I still have not hooked up the Master Volume, so for now I am bypassing that and feeding the 220k Grid Load Resistors as normal. The one installed is a 1M, I just put it in as a place holder so I could make sure I have enough room. I have the 250K ganged pot on order, but it seems to be back ordered for now. Hopefully I will get it soon. Along with some knobs that are on order as well. Then to finish with the face plate.




Tubed up and ready to be played.



Here is a copy of the schematic for anyone interested. I like to create nice schematics when I get to a good stopping point. That way if I decide to do some mods, it is easy to keep the schematics updated with versions.



What started as a mystery amp is now a very promising design that is capable of hanging with the band. This thing has serious balls.

Craig

[sgelectric]

Really nice work. Thanks for sharing....SG

[Bruce / Mission Amps]

For anyone still interested, I have completed 99% of the amplifier build. The chassis is extremely tight using 3-12AX7's, 2-6L6GC's and a 5U4GB Rectifier tube. I went with all point-to-point construction for the most part. There is very minimal low-level hum, which I suppose is to be expected in this kind of build, I will try to clean it up a little at a later date. The sound of the amp is very nice. It is very loud and crystal clear up until about 9'oclock on the volume, then it gradually just builds up it's roar. Very tweed sounding. It has plenty of low end, without getting farty and chimes to match. Cleans don't go too far up the dial, but that is ok, I have clean amps. I have never played a 6L6 cathode-biased amp before, I really like the way this thing responds to the guitar volume and pick attack. Frankly, I am pretty stoked.

I added a switchable "crunch" switch to the pre-amp, which is basically just switching both triodes of V1 into a parallel out configuration. This provides a little more warm meat to the output without going into death metal territory. With the crunch switch engaged, this thing really roars to the point of sounding very plexi-like. And loud. But manageable.

So here is a photo of the innards...I added the 470 ohm Screen Grid Resistors. I still have not hooked up the Master Volume, so for now I am bypassing that and feeding the 220k Grid Load Resistors as normal. The one installed is a 1M, I just put it in as a place holder so I could make sure I have enough room. I have the 250K ganged pot on order, but it seems to be back ordered for now. Hopefully I will get it soon. Along with some knobs that are on order as well. Then to finish with the face plate.




Tubed up and ready to be played.



Here is a copy of the schematic for anyone interested. I like to create nice schematics when I get to a good stopping point. That way if I decide to do some mods, it is easy to keep the schematics updated with versions.



What started as a mystery amp is now a very promising design that is capable of hanging with the band. This thing has serious balls.

Craig

You better watch that second stage parallel triode with the switch open... you have no bias voltage with no grid load. And with it open... I suspect you have a large voltage drop across the common, 100K plate resistor.

[Csquare4]

You better watch that second stage parallel triode with the switch open... you have no bias voltage with no grid load. And with it open... I suspect you have a large voltage drop across the common, 100K plate resistor.

Bruce, I see what you are saying with that. Thanks for pointing that out. I will fix that as well. I guess I am surprised it has worked as long as I played it the last few days.

[jmaf]

I guess I am surprised it has worked as long as I played it the last few days.

No reason to be surprised, really, as it works fine like that - but it's probably not what you intended to do(use that 12ax7 as a diode to lower +B when the grid is floating).

Suggestion: The grid circuit in V1 is always the most sensitive to noise and oscillation, because it controls all the rest of the amp. If you leave both grids connected as it is now, but instead switch a capacitor on each cathode, you'd probably get crunch / clean as well. Avoid switching grids whenever possible. With a dpdt switch you can switch both cathode caps at the same time.

[Csquare4]

No reason to be surprised, really, as it works fine like that - but it's probably not what you intended to do(use that 12ax7 as a diode to lower +B when the grid is floating).

Actually, I went to check that this morning and realized that I had originally tied a 1Mohm resistor between the switch/grid and ground as well, I had missed it when I drew the schematic...so that was why it was working so well I suppose. The updated schematic looks like this:

Suggestion: The grid circuit in V1 is always the most sensitive to noise and oscillation, because it controls all the rest of the amp. If you leave both grids connected as it is now, but instead switch a capacitor on each cathode, you'd probably get crunch / clean as well. Avoid switching grids whenever possible. With a dpdt switch you can switch both cathode caps at the same time.

So let me see if I understand what you are saying...you suggest I should try it out leaving both grids on V1 tied together, but add a bypass cap across the 1.5k cathode resistor and put a DPDT switch on it and the existing 25uF bypass cap so that they are either both on or both off through the switch? That may be something I can try if I understand it correctly. Any reason not to use the same 25uF value?

[jmaf]

So let me see if I understand what you are saying...you suggest I should try it out leaving both grids on V1 tied together, but add a bypass cap across the 1.5k cathode resistor and put a DPDT switch on it and the existing 25uF bypass cap so that they are either both on or both off through the switch? That may be something I can try if I understand it correctly. Any reason not to use the same 25uF value?

Exactly. Try it, it's quieter as it's done near ground and far away from the grids. See if the gain increase suits your clean/crunch switching needs.

Edit: No reason not to try the 25uF value btw but you can experiment with lower values if it becomes too boomy, that cap can help you shape the tone of the gain stage very efficiently in my experience.

[Csquare4]

Exactly. Try it, it's quieter as it's done near ground and far away from the grids. See if the gain increase suits your clean/crunch switching needs.

Edit: No reason not to try the 25uF value btw but you can experiment with lower values if it becomes too boomy, that cap can help you shape the tone of the gain stage very efficiently in my experience.

Alright, that should be easy enough. I will check it out to see how it works. Thanks for the input!

[Csquare4]

Latest iteration based on jmaf's suggestion above. I hardwired the two grids of V1 in parallel and added a switch on both cathode bypass caps. I stuck with the 25uF on the V1A and went with a 2.2uF on the V1B side through a DPDT switch. So this is what this version looks like:



I finished this late so I really haven't had a chance to turn it up yet. With the volume at about 9 o'clock and playing through an attenuator that was way down in the mud, it sounded pretty good on the bridge pickup and pretty muddy on the neck with humbuckers. Obviously, the attenuator will affect that and I didn't really get a chance to tweak the tone stack at all, but I think I may like this. It may get brittle with single coils, I need to check that as well. Played some Trower Bridge of Sighs at low volume and it has some serious sustain. I may tweak the caps some more depending on how it sings with some real air moving. Right now, I think it might just work.

Thanks again for all the suggestions on this build.