Hammond "JCM800 Replacement" choke

[azrael]

I ordered this a little while ago, thinking it would be okay for my Peavey VTM60. The VTM is a close relative of the JCM800.

http://www.tubesandmore.com/products/P-C194C

However, when I received it, I found out that it was 40H and rated for 50mA. Still, I thought, if it's marketed as a JCM800 drop in replacement, it should still work. Measured the DCR as 418 ohms.

I converted the amp to be able to use EL34s, currently running E34Ls in it.

I attached my current schematic, I think I updated everything I changed.


So, I tried the choke in place of R4 on the power supply board. The plate voltage drops from 470V to 330V. The bias current is super low, I probably could have biased it up correctly, but I didn't want to mess with the voltage divider.

Everything works fine again if I replace R4 with the original 400 ohm resistor.

Thoughts on what's going on? I'm a little peeved, since AES' description is misleading if it doesn't work right. There should have been more info on their product description.

[Tillydog]

Hopefully you mean R5, not R4.....

Check that plate supply (B++) still comes off the main reservior caps (C3/4) and that nobody has moved it to after the standby switch (B+).

0.02p

[roberto]

Post some pics of the mods, we'll find the problem.

[azrael]

No one has modified this amp except for me, I have not moved the B++.

And yeah, I meant R5, sorry.

Pics of all the mods? Kind of a hassle with all the multiple circuit boards.

Again, if I reinstall the original resistor, the amp functions normally, sounds great. I don't think any of the additional mods are part of this new problem, I've been using this amp for years, the choke is a more recent mod.

[Structo]

When I look up the specs for a JCM800 50w choke I get 5 henries at 120mA DC. 115 ohms DC resistance.

So quite a bit lower resistance.

[azrael]

Well, the original dropping resistor in the VTM is a 400 ohm one. But it shouldn't be a big different anyway.

[roberto]

This is the reason why I asked to add the pics of the choke installation.
Have you measured it's 420Ohm? Have you seen R5 is 400Ohm? Then it could not be 330V instead of 470V. It's 140V drop for something around 20mA (screens and preamp), the choke should be 7k4 Ohm.

50-60mA choke is ok for a 50W amp, what's unusual is raising inductance and resistance at the same time (like a Vox), maybe to obtain more sag at lower volume levels?

[Roe]

a marshall type choke should be around 3h, 110ohms, 100+ma

[azrael]

This is the reason why I asked to add the pics of the choke installation.
Have you measured it's 420Ohm? Have you seen R5 is 400Ohm? Then it could not be 330V instead of 470V. It's 140V drop for something around 20mA (screens and preamp), the choke should be 7k4 Ohm.

50-60mA choke is ok for a 50W amp, what's unusual is raising inductance and resistance at the same time (like a Vox), maybe to obtain more sag at lower volume levels?

Yes, I said that I measured the DCR to be at 420 ohms.

I have not measured the original resistor, but on the schematic it is marked as 400 ohms, and that's what it says on the resistor itself.


I know it doesn't make sense, that's why I'm posting it.
The choke is no longer in the amp, I need the amp functioning properly for gigging, which is why I placed the original resistor back in.


How do you calculate the screen current?

[xtian]

How do you calculate the screen current?

Measure voltage drop across screen resistors and divide by resistance.

[azrael]

Oh, of course. I was thinking there was a day to do it theoretically, hehe.

I'm not always by my amp, so ya.

[roberto]

It was useful to have some pics of the choke installed, to try to understand what was wrong. The only possibility it's that was mounted incorrectly.

About the 20mA, it's just something around 5mA each screen + 10mA preamp, with some approximation.

[azrael]

Well, it was literally just installed in place of R5. Just replaced it.


Mounted improperly? Like, mounted to the chassis improperly? This is possible?

[Reeltarded]

No, of course not.


[IMG:500:375]http://i273.photobucket.com/albums/jj20 ... e_fail.jpg[/img]

[Badside]

Always found the specs on that choke a bit baffling. Interestingly, it's not listed on Hammond's website anymore.

When I built my JCM800 clone, although I used their "replacement" line for the PT and OT, I went with a generic choke (159Q, 7H, 150ma rating) which seemed more in line with the original specs. Was cheaper too!
You could always return it and get one from the 159 series as well.