New Build, a mutt.

[NickK_chugchug]

I was comparing the original jcm800 (12ax7 PI - el34 pwr) and from your schematics with the 12ax7 PI-ef80 pwr. If the original doesn’t have issues the same issues (perhaps due to grid capacitance), but there does seem to be common theme with stability.
Good you have that solved!

No worries, will tech-back the suggestions in future

 

[mountainhick]

I was comparing the original jcm800 (12ax7 PI - el34 pwr) and from your schematics with the 12ax7 PI-ef80 pwr. If the original doesn’t have issues the same issues (perhaps due to grid capacitance), but there does seem to be common theme with stability.
Good you have that solved!

No worries, will tech-back the suggestions in future

This project has nothing to do with JCM800. My schematics do not use EF80s.

You are confusing this with someone else's project.

I do appreciate your input, but prefer the discussion stay on topic.

 

[printer2]

Thanks for taking the time. Hope you are feeling better.

How about the power tube grid stopper wiring? Maybe better to switch the lead wires with the resistors? i.e. place resistors from board eyelets to tube pins?

Feeling better, thank you. I tend to put the stoppers right at the pins. Not a great picture, a 6AK5 output tube 5E3.

Since you are going in and messing about I would shorten a few wires. The yellow on the cathodes of the 6AQ5's could be shorter and tighter loop around the right socket. Next if she still sings to herself in the shower I would also put the screen resistors closer, Maybe on a standoff mounted close to the sockets. We are spoiled by octals allowing us to put both grid and screen resistors on the sockets. At this point if you still have the issue I would disconnect the heater elevation. It should not matter but then you should not be having this issue without the preamp tubes. If you do mount the screen resistors closer maybe you could use the old location to install some 1 ohm resistors there to measure the bias current of each tube? It does not have to be 1 ohm, I have used 10 ohm before, just watch the digit when calculating. Good luck, I'll be watching.

 

[mountainhick]

Feeling better, thank you. I tend to put the stoppers right at the pins. Not a great picture, a 6AK5 output tube 5E3.

Since you are going in and messing about I would shorten a few wires. The yellow on the cathodes of the 6AQ5's could be shorter and tighter loop around the right socket. Next if she still sings to herself in the shower I would also put the screen resistors closer, Maybe on a standoff mounted close to the sockets. We are spoiled by octals allowing us to put both grid and screen resistors on the sockets. At this point if you still have the issue I would disconnect the heater elevation. It should not matter but then you should not be having this issue without the preamp tubes. If you do mount the screen resistors closer maybe you could use the old location to install some 1 ohm resistors there to measure the bias current of each tube? It does not have to be 1 ohm, I have used 10 ohm before, just watch the digit when calculating. Good luck, I'll be watching.

Thanks, I do this for input grid stoppers with terminal strips. I am not too fond of the technique of suspending the resistors off the tube pins shrink wrapped to the wire, but may have to resort to that here. Works, nothing wrong with it really, just personal choice. I did this with the mixing resistors on this project at the FX loop.

This brings up other related questions, I asked part of it earlier:

-Do mixing resistors merging 2 channels count as grid stopper to the next stage?
-In my case with this pair attached to the send FX loop jack, should I add another grid stopper at the PI tube input V3A?
-Should I also add another grid stopper on V3B?

It was a conscious decision this time around to allow a little more length in the tube pin wires to ensure they are truly on the bottom against chassis and assist ease of wiring. The stranded wire is not the easiest to get to lie flat. I will revisit this notion.

Question here too:

I also was a little more generous for screen and some other plate wires, like V1A... same concern, or are grid and cathode wires more sensitive?

 

[printer2]

Agree with you on hanging parts in the air, stranded wires. Not something I really like doing but not the end of the world for a home amp. Would not do it for an amp that I think someone will make their living with. Mixing resistors can act as grid stoppers in terms of slamming the next input grid, not to stop RF interference though. Which reminds me, wanted to say it yesterday but forgot. Sometimes you can get interference with the case open but once you button everything up your problem goes away due to the shielding. @Jerry garrcia found that out in one of his builds if memory serves me right. You do not have a drawing with the effect loop, when in use you would have the mixing resistor on the wrong side of things to act as a filter for the PI.

What I have done at times is just put a little stopper on the input of the grids (as in the above picture) with maybe 10-20k. Not high enough to effect the circuit operation but gives a some protection if the local environment has a noise source (say a noisy computer, lot of crap can come off the keyboard or monitor). On the second PI input, I do not think I would put a stopper in unless you find that is a source of problems. On the length of wires, I do not like leaving more than really necessary. The longer the wires the greater the chance they can talk to each other. On which are more sensitive you look at what the impedance to ground is. With a cathode resistor of 1.5k you are getting into almost a short circuit to ground, add a bypass cap and the AC thinks the cathode is grounded. On the plate you have (say) 100k in between the plate and ground so it would be next in line as far as sensitivity. With the grid, there is Meg ohms of impedance for a signal to get through to get to ground. That is why you would want coupling caps to have the outer foil (if the cap is built this way) to be facing the previous stage where the electrode that is attached to the inner foil goes to the high impedance grid.

 

[mountainhick]

Agree with you on hanging parts in the air, stranded wires. Not something I really like doing but not the end of the world for a home amp. Would not do it for an amp that I think someone will make their living with. Mixing resistors can act as grid stoppers in terms of slamming the next input grid, not to stop RF interference though. Which reminds me, wanted to say it yesterday but forgot. Sometimes you can get interference with the case open but once you button everything up your problem goes away due to the shielding. @Jerry garrcia found that out in one of his builds if memory serves me right. You do not have a drawing with the effect loop, when in use you would have the mixing resistor on the wrong side of things to act as a filter for the PI.

What I have done at times is just put a little stopper on the input of the grids (as in the above picture) with maybe 10-20k. Not high enough to effect the circuit operation but gives a some protection if the local environment has a noise source (say a noisy computer, lot of crap can come off the keyboard or monitor). On the second PI input, I do not think I would put a stopper in unless you find that is a source of problems. On the length of wires, I do not like leaving more than really necessary. The longer the wires the greater the chance they can talk to each other. On which are more sensitive you look at what the impedance to ground is. With a cathode resistor of 1.5k you are getting into almost a short circuit to ground, add a bypass cap and the AC thinks the cathode is grounded. On the plate you have (say) 100k in between the plate and ground so it would be next in line as far as sensitivity. With the grid, there is Meg ohms of impedance for a signal to get through to get to ground. That is why you would want coupling caps to have the outer foil (if the cap is built this way) to be facing the previous stage where the electrode that is attached to the inner foil goes to the high impedance grid.

Gold nuggets! Thanks!

 

[Jerry garrcia]

Sometimes you can get interference with the case open but once you button everything up your problem goes away due to the shielding. @Jerry garrcia found that out in one of his builds if memory serves me right.

As always your memory serves you well. Happened to me.
/angry man waiting for parts and really hates the Swedish customs and have a ton of transformers that I need to measure up but don’t want to do since it is so time consuming. And my 21 h sourdough bred didn’t rise as expected. . Only fun thing is that I’m digging in to the subject if I can use a 220V primary power transformer as a push pull OT since it has a 110V tap. Need to get a life

 

[mountainhick]

Like with the Bobcat, I am finding that it seems the 6AQ5 indeed runs better at the spec 250V rating. I am having the same problem with this amp since the HV is ~300V.

But whereas I have some leeway with power supply voltage in the Bobcat, I am at the lowest with this PT and 5Y3 without adding extra sag which it doesnot need! So I revisited both heater and HV current ratings for this old Sparton radio PT by adding up all the radio's tube values, and it will run a pair of EL84s... upgrade coming up! I can also goose the HV with the 5V4.

So is there a difference between 6AQ5 and EL84 grid input voltage sensitivity?
Will I need to adjust?

Oh, and the tube spacing is pretty close, any problem with the EL84 pair being on 1.5" centers, 3/4" in in between

 

[mountainhick]

Alright! I am officially calling this THE MUTT now.

It started from a Gibson GA-40 design but has morphed several times. Now much better with a pair of 6BQ5s than the 6AQ5s. I had it on for an hour and the PT is warm to the touch, but can hold on to it no problem, so I think it will hold up. It is sounding very good with both single coils and humbuckers, but top quarter of the volume sweep, it enters some unpleasantness. I started scoping it, and looks like the PI sides are quite different, so thinking about how to balance them better. The dummy load I have is only 20W and heated up, so waiting for 100W resistor to resume. Should be here soon.

I think I have the load line and bias in a good place, would appreciate more experienced eyes than mine double checking!

Voltages with the 5V4 rectifier, and a pair of SOVTEK EL84s

B+328V
180 ohm cathode resistor, Cathode 12.4V
323 on the plates, net 315.6
322 on the screens, net 314.6

 

[YellowBoots]

PSA: Anyone posting VTADIY load lines, please hover your mouse over the 0V grid line when you screen shot so we can see which one it is. We can’t tell much unless we see where 0Vgk is. Thank you. Resume normal programming.

 

[mountainhick]

PSA: Anyone posting VTADIY load lines, please hover your mouse over the 0V grid line when you screen shot so we can see which one it is. We can’t tell much unless we see where 0Vgk is. Thank you. Resume normal programming.

Done.

 

[printer2]

PSA: Anyone posting VTADIY load lines, please hover your mouse over the 0V grid line when you screen shot so we can see which one it is. We can’t tell much unless we see where 0Vgk is. Thank you. Resume normal programming.

I was wondering the same thing when looking at another set of curves posted, good idea to give at least one way point.

 

[YellowBoots]

Done.

If you scope the screen voltage during overdrive, can you see how much it is sagging? I’m curious how much you are getting with only 470R resistors and choke.

 

[mountainhick]

If you scope the screen voltage during overdrive, can you see how much it is sagging? I’m curious how much you are getting with only 470R resistors and choke.

I looked at a bunch of schematics with cathode biased 6BQ5, and was surprised to see some with only 100ohm screen resistors. I decided to increase them to 1K 2W. Grid stoppers are 5.6K.

I got the 100W 8ohm load resistor, need to cobble it up as dummy load.

Just getting used to good scoping practices, I guess 100X probe for screen voltage?

 

[YellowBoots]

Just getting used to good scoping practices, I guess 100X probe for screen voltage?

I’m no expert with a scope, but I would think a 10X would be all you need. Just check the specs on your equipment for voltage ratings. You probe needs to be able to withstand at least 600V…. again check the ratings. When you probe the screen voltage, you can leave it in AC coupling and just measure the amplitude of the dips. More than one way to do it.