Changed Phase Inverter values, amp significantly less loud.

[Gainzilla]

Hi all,

I've got this Jet City JCA20HV, which is the Joe Perry version of the 20h. Cool little amp. I've done some various mods, primarily just upgrading caps, tweaking values here and there, upgrading tubes, biasing, etc. The entire time, however there was harsh, ice-picky high end that gets worse the louder I turn the master volume.

I realized (I think) that this amp had a standard Marshall style Phase Inverter, which from everything I've read, pushes the el84's too hard. To get to the point, I swapped out the PI values to reflect 18 watt values (820 bias, 47k tail, 470k grid to ground resistors). The result, MUCH nicer tone. For the first time this amp has the tone I think it should've had, even at loud volumes.

BUT, now it doesn't get nearly as loud. Maybe a third less. Part of me feels like its a worthwhile tradeoff, but the other part still wants to be able to play this amp with a sober drummer.

Do the PI values effect the overall volume that much (silly question)? If so, would changing the 470k grid to ground resistors back to 1M, and bumping the el84 input grid resistors to 8.2k get me some volume back while keeping the nice tone I've got now intact?

Schematics attached for your convenience.

Thanks in advance!

[LOUDthud]

How does the B+ at the phase inverter compare to the 18W lite schematic? 47K seems kind of high to me, but it could just be that the 820 biases the PI too cold. Try lowering the 820 a little at a time. 750, 680, 620 etc. or just stick in a 1K trim pot.

[Gainzilla]

Thanks for the reply! I didn't get the voltage last night. BUT... I entered the values from the schematic into Duncan's PSU Sim (Transformer spec is right on the PT, so that helped), compared that to the voltage I was getting while setting the bias. Looks like the PI SHOULD be a shade higher than the 18watt schem at 337v (I'll verify tonight). Assuming that's true, any thoughts?

Great idea about the trimmer. I think I have one at home, but if I don't I should have a couple smaller caps to try.

Thanks again!

[xtian]

+1 on reduce the tail resistor from 47k to 33k or 27k.

[Cameron]

Hi all,

I've got this Jet City JCA20HV, which is the Joe Perry version of the 20h. Cool little amp. I've done some various mods, primarily just upgrading caps, tweaking values here and there, upgrading tubes, biasing, etc. The entire time, however there was harsh, ice-picky high end that gets worse the louder I turn the master volume.

I realized (I think) that this amp had a standard Marshall style Phase Inverter, which from everything I've read, pushes the el84's too hard. To get to the point, I swapped out the PI values to reflect 18 watt values (820 bias, 47k tail, 470k grid to ground resistors). The result, MUCH nicer tone. For the first time this amp has the tone I think it should've had, even at loud volumes.

BUT, now it doesn't get nearly as loud. Maybe a third less. Part of me feels like its a worthwhile tradeoff, but the other part still wants to be able to play this amp with a sober drummer.

Do the PI values effect the overall volume that much (silly question)? If so, would changing the 470k grid to ground resistors back to 1M, and bumping the el84 input grid resistors to 8.2k get me some volume back while keeping the nice tone I've got now intact?

Schematics attached for your convenience.

Thanks in advance!

Do you still have feedback from the speaker? The 18 watt does not...and the PI is not designed to have it...as drawn.

[frankdrebin]

Is the new circuito cathode bias?
The schemes have many differences,including feedback which is very important.

[Gainzilla]

Thanks for the input guys (pun intended).

Ok, quick update: here are the voltages:

V3a
300v B+
232.2v into anode (ie after 82k)

V3b
300v B+
228.3v into anode (ie after 100k)

- Amp is fixed bias
- 25k presence pot
- .68 presence cap (changed this from the stock .1)

[Edit - Forgot to mention this:]
NFB is coming from the 8 Ohm tap.

After reading up a bit more, I realize the whole string from the bias to presence pot/cap influence the overall design. A bit lost on how to calculate that all though. If you were to design this PI, what might your thinking be? I'm totally up for changing everything if need be. I'm after good tone and reasonable volume, but overall not punishing the el84 into shrieking at me.

Cheers!

[Gainzilla]

I was trying to figure out why xtian suggested I should lower the tail resistor, but I think I see now. Is it because the total of the bias resistor, tail resistor, and presence resistor to ground all add up to like 52k, and that by reducing the tail to 33k the PI would see a more reasonable 38k?

I've been doing a bunch of reading on how to calculate PI values, but the thing that isn't clear is how you optimize it for driving a specific set of tubes (PI optimized for driving a pair of el84s without making them scream in pain, or some balance between volume, tone, and pain? lol).

Anyway, Later variations of the AX84 November uses a p/p pair of el84s, with negative feedback. The Values are:

82k/100k anode
1M/1M Grid
470 Bias
10k tail
(4.7k Presence/47n cap. Does not have the same resistor to ground that the JCA20HV has)
100k/100k el84 grid leak resistors

However, Steve Luckey's earlier variant has the following:
82k/100k anode (CORRECTION: actually 15k/15k)
1M/1M Grid
470 Bias
47k tail
(4.7k Presence/.1uf cap.)
100k/100k el84 grid leak resistors

Thoughts on this? Thanks again for the help!

Cheers!

[sluckey]

However, Steve Luckey's earlier variant has the following:
82k/100k anode

My November is an exact copy of Revision 11 found on ax84.com. PI anode resistors are 15K, not 82K/100K.

I recently built Mark Huss's Plexi 6V6 and used the same values Mark used (82K/100K)

[Gainzilla]

Thanks Steve! Wow, I totally missed that. I've never seen PI values like that before. Why so low? We're you happy with those values in the amp? How did the overall tone/volume seem? Sorry for so many questions.

Cheers