Help needed verifying plate dissipation/Cathode resistor value for 6V6. In other words, check my work please!!

[Michael Inglis]

The amp is a MOD102+ that i am adding a cathode lift switch to select between the stock EL84 and a 6V6. I already have that figured out. What im trying to verify is my cathode resistor and cap value. Here is the schematic with the voltages.

EDIT: I just realized i was looking at the schematic for the MOD102 not the MOD102+ so ill post the MOD102 schematic and if my numbers are right then ill know how to do it for the voltages on the MOD102+





For the EL84 i used the tube dissipation calculator and im getting 8.8 watts which seems right cause they say this amp puts out 8 watts. Here is that calculation.




Now i need to get my cathode resistor values figured out so i entered everything the same except changed the tube type to 6V6 and got the following. But would my voltages change with the 6V6? In other words would i need to take new readings with the 6V6 or would they be the same voltages as with the EL84?





I know D-Lab on you tube did a conversion on this kit to a 6V6 and then he listed it on reverb. From the pictures on reverb i can see he used a 470R resistor and when i plug those values into the calculator i get 2.8 watts which makes sense cause in D-labs listing hes saying it puts out about 3 watts. If your able to help me with this i would be VERY GREATFUL. Thanks in advance!!

 

[Jon Snell]

A 6V6 or any other output valve requires a lot more than 30mV drive on the grid to attain full output.
5.5vDC on the cathode is far too low. The cathode resistor should be about 270 Ohms to get 22vDC or about 70mA.

Perhaps you ought to measure again.

 

[Michael Inglis]

A 6V6 or any other output valve requires a lot more than 30mV drive on the grid to attain full output.
5.5vDC on the cathode is far too low. The cathode resistor should be about 270 Ohms to get 22vDC or about 70mA.

Perhaps you ought to measure again.

These are the measurements with the el84 in the v2 position, would my voltages change with the 6V6 in v2? Cause if they do I can remeasure once the amp is built and I have the 6V6 in and rerun my calculations. I'm just going off the voltages that I have with the original schematic. This is VERY helpful so thank you! I know it's a PITA but any explanation you can offer would be an invaluable learning experience for me right now!

EDIT: If what you say is true I must be doing something wrong with the calculation values? You say there isnt enough current on the grid, screen grid or control grid? And how are you getting 30ma? Bear with me, I'm teaching myself and just now learning to set cathode bias values. My main amp is a DSL100HR/fixed bias which I figure out my plate dissipation with plate voltage and I run it around 65% plate dissipation for a good balance of tone and tube longevity. But cathode bias is still very new to me.

 

[Marcomel79]

In a cathode bias type amp, as opposed to fixed bias, the plate dissipation is calculated at 100%. So 8.8W plate dissipation for a 6V6 is far too low...

 

[Michael Inglis]

These are the measurements with the el84 in the v2 position, would my voltages change with the 6V6 in v2? Cause if they do I can remeasure once the amp is built and I have the 6V6 in and rerun my calculations. I'm just going off the voltages that I have with the original schematic. This is VERY helpful so thank you! I know it's a PITA but any explanation you can offer would be an invaluable learning experience for me right now!

In a cathode bias type amp, as opposed to fixed bias, the plate dissipation is calculated at 100%. So 8.8W plate dissipation for a 6V6 is far too low...

This is exactly what i thought but since the stock EL84 is set at 5watts (MOD102) and 7.8 watts (MOD102+) i just figured they were supposed to run a little low. But i did know that Class A Cathode bias should be closer to 100% dissipation. But is my calculations correct? I just want to make sure im doing it right then i can redo the resistor values with the voltage on the schematic to get the ideal plate dissipation.

 

[Marcomel79]

This is exactly what i thought but since the stock EL84 is set at 5watts (MOD102) and 7.8 watts (MOD102+) i just figured they were supposed to run a little low. But i did know that Class A Cathode bias should be closer to 100% dissipation. But is my calculations correct? I just want to make sure im doing it right then i can redo the resistor values with the voltage on the schematic to get the ideal plate dissipation.

If you follow the schematic and your voltages match the ones in the schematic, you should be ok

 

[Michael Inglis]

If you follow the schematic and your voltages match the ones in the schematic, you should be ok

Ok so maybe you can help me understand this. I thought a single EL84 in single ended configuration class A could only put out a max of 6w because its amplifying both halves of the cycle rather than just one half like in a push pull amp with 2 output tubes. But if i bias this amp to 100% dissipation or very close to it then im supposedly getting 11.9. I know that an EL84 can put out 12w and a 6V6 can put out 12w (14w for a JJ 6V6GC) but to get that full 12w i thought it had to be in push pull configuration. What am i missing here? Looking at the schematic above for the MOD102 with the voltages they show im getting only 6.6 watts (and they claim that amp to be about 5 watts). And even with the updated version of the amp (MOD102+) im only getting about 7.8 watts. So have they designed these amps with too cold a bias or am i doing the calculations wrong/missing something.

EDIT: Ok so i see now that Plate Dissipation wattage doesnt equal Output wattage.

 

[Pete Farrington]

I don’t know anything about the mod circuits being referred to. But you seem to be getting an amp’s audio power output mixed up with a valve’s anode dissipation limit. Sorry if I got the wrong end of the stick.

 

[Marcomel79]

Ok so i see now that Plate Dissipation wattage doesnt equal Output wattage.

Exactly

 

[Michael Inglis]

I don’t know anything about the mod circuits being referred to. But you seem to be getting an amp’s audio power output mixed up with a valve’s anode dissipation limit. Sorry if I got the wrong end of the stick.

Pete.....i was SO hoping you would chime in. And you are correct. I edited the reply once i realized that.

The MOD102 and MOD102+ is a parts kit sold by amplified parts thats pretty much a fender champ with solid state rectification and an EL84 instead of a 6V6. I went for it for my first amp build mainly because of cost. Im just trying to work out the bias resistor and bypass cap values for a 6V6 in this circuit because Im modding it so that it can run either an EL84 (like it was designed to) or a 6V6. To do that im just adding an octal socket and wiring it in parallel with the noval socket (minus the cathode pins) and using a switch to lift the cathode of one tube at a time. This is how the Mesa Boogie Blue Angel amp accomplished switching between EL84's and 6V6's.

In the schematic for the MOD102 (ill repost it under this reply) it shows 5.5 vdc on pin 3 and ive been assuming this was the cathode resistor voltage drop but im not entirely sure that is correct? I have another sheet which gives the voltages for all the pins with the EL84 in place and it also says 5.5 vdc. But if thats not the voltage drop i dont know what it would be? So if i use 5.5 for the voltage drop with a 200R resistor (the value used in the MOD102 schematic) im only getting 6.6 watts for the dissipation. Wouldn't that be too low?

The MOD102+ changes the cathode resistor value to 150 and updates the voltages on the schematic and if i run those numbers i get 7.9 watts which still seems too low. So should i change the resistor value to get closer to 100% dissipation?

Im sure your busy Pete but any clarification you could offer would be very helpful. Im trying my best to wrap my head around all this but im still not there yet lol.


MOD102 Schematic 6.6 watts plate dissipation?




MOD102+ Schematic, 7.9 watts plate dissipation?

 

[Pete Farrington]

The idle dissipation should be determined by the loadline. Class A loadlines typically require high idle dissipation for centre bias, but the low HT voltage here isn’t typical.
Given the OT has a 5k primary impedance, the lower HT will typically require a lower current for centre bias, hence lower dissipation.

Generally, first impressions are why is the power switch in the neutral feed? It surprises me how people publishing this stuff don’t perceive any liability?

 

[Michael Inglis]

The idle dissipation should be determined by the loadline. Class A loadlines typically require high idle dissipation for centre bias, but the low HT voltage here isn’t typical.
Given the OT has a 5k primary impedance, the lower HT will typically require a lower current for centre bias, hence lower dissipation.

Generally, first impressions are why is the power switch in the neutral feed? It surprises me how people publishing this stuff don’t perceive any liability?

Wow! i hadnt even noticed the power switch being on the neutral wire! Im no expert but even i learned about that when modifying my variac awhile back. If i remember correctly you want to switch the hot wire so the circuit isnt hot in case of a short etc.? So i would assume a better course of action would be to swap the power switch to before or after the fuse on the hot wire? If i were to take a complete shot in the dark about why they did it that way i would say maybe it was to reduce the possibility of blowing fuses on power on since they chose to use a fast blow fuse type?

On an actual Fender Champ they switch both the neutral and hot wire simultaneously and also use a slow blow fuse. So again its strange they changed that for this amp. Youd think they must have had some logic behind their decision....youd HOPE they had some logic behind that decision lol.

 

[Pete Farrington]

6V6 need about double the bias voltage of an EL84. Without redesigning the whole thing, I think the best way forward may be to use a dual pole mains switch, and try a 6V6 cathode resistor about twice the value of the EL84 cathode resistor.
The bypass cap value could be about half that of the EL84, but it’s probably not a critical thing.

 

[Michael Inglis]

6V6 need about double the bias voltage of an EL84. Without redesigning the whole thing, I think the best way forward may be to use a dual pole mains switch, and try a 6V6 cathode resistor about twice the value of the EL84 cathode resistor.
The bypass cap value could be about half that of the EL84, but it’s probably not a critical thing.

Excellent, thats not too far off from the 470R 5w id planned on so ill use another resistor in parallel to drop it down a bit more. Especially considering the version im building already dropped the EL84 bias resistor down from 200 to 150 so if i were to double it based on the new 150 cathode resistor double would be 300 which is a fair ways off from 470.

Also thanks for the bypass cap recommendation, i was still fairly lost on that. All ive gathered so far is too low a bypass cap and you start cutting bass so it seems like a common mod on Champs and similar amps from that time period is to raise the bypass cap if they were set low and cutting frequencies in the audible range. Probably not an issue with those 8 inch speakers but ill be using this amp mainly with two orange 1x12's that are actually capable of hitting those lower frequencies.

Now for that dual pole mains switch suggestion. Im actually planning on using a dual pole on on off type switch so i can have a standby setting (with a separate on/on for the cathode lift tube selector. I thought about getting another on on on switch instead and using it as a 6V6, Standby, EL84 with a separate on/off switch. But i suppose i could also just ditch the standby setting and use just the on on on for a 6V6, off, EL84 set up. Was that what you were alluding too or am i way off?

Thanks for the help Pete!

EDIT: Oh lol, i bet you meant a dual pole to switch the hot and neutral at the same time like we discussed previously about the Champ schematic lol! Went right over my head.

 

[Pete Farrington]

i bet you meant a dual pole to switch the hot and neutral at the same time like we discussed previously about the Champ schematic lol!

Haha, yes, sorry I should’ve been more clear

 

[Michael Inglis]

No worries, it was plenty clear lol! Ive got a graduation to go to this weekend at UT in Austin so im putting of the build until next week. But i plan to chip away at it after work each night and likely have it finished by the following weekend. If i were to build it stock i imagine it wouldnt take more than a couple hours but since im changing some stuff up and adding some things here and there ill likely go slow and take my time with it. Thanks again for taking a look at it for me!