Jet City JCA20 (Soldano Astroverb 16) questions.

[nandrewjackson]

I have a jet city combo that is basically a Soldano Astroverb, but made in China.  It's a really nice amp and I recommend Jet City products, you can't beat the price. They are all designed by Mike Soldano, made in China for cost reasons.
It's a single 12" speaker, 2- EL84 powersection, mid to high gain combo.
Anyway,

I've looked through the circuit as compared to the Astroverb schematic I'm posting here, it's about 85 percent identical.

I would like to get more Reverb out of it.  Even with the reverb control turned all the way up, it is still not much reverb.
I'm not familiar with the way reverb circuits operate.
I changed the 15K from V4B grid to ground to a higher value thinking that would raise the amount of reverb, but I was wrong apparently....

[jazbo8]

V4b's plate and cathode resistors seem odd, are they not backwards?

[nandrewjackson]

that schematic is a bit goofy here and there. I found it online.  MOST** of the circuit is similar to the Jet City amp, and offhand I don't know the values on the JET City, but you're probably right about those 2 values being backward.

[sluckey]

There are several specific things that can be done to that Astroverb to increase reverb. Without seeing your actual schematic I can't say that these specifics will apply to your amp, but it will at least give you some direction...

First the driver V4a. Put a 22µF cap across the 1KΩ cathode resistor. Then replace that 10K/3W plate resistor with a wire jumper.

Now the recovery V4b. Change the 15K grid resistor to 220K. Maybe even short out the 10K grid resistor. Change the 10K plate resistor to 100K. Change the 100K cathode resistor to 1.5K. Change the 1µF cathode bypass cap to 22µF. Replace the 47K that connects to the top of the reverb pot with a wire jumper. Remove the 220K resistor that parallels the reverb pot.

All the above will increase the reverb signal level, but the two components that will change it the most will be V4b plate and cathode resistors. I'd do those first. That could just be all the extra reverb you want. These changes will make the astroverb reverb circuit closely resemble Tubenit's one tube reverb, and that's plenty of reverb for most people but it's not Dick Dale verb.

[nandrewjackson]

Thanks, Sluckey,
I've got the chassis out but it may be a while before I get to making the changes,
I'll keep it posted here, and I'll try and get the values of the JetCity written down to compare to that schematic of Astroverb.

I know the first time I went through the circuit there were quite a few differences, but I didn't write it all down.  That's why I gave my guestimate of 80-85% similarity.

[nandrewjackson]

I snipped out the resistor parallel to the reverb pot and that pretty much did the trick!

Thanks!

[tubenit]

Sluckey gave you some excellent suggestions to increase the reverb!

Take a look at this for comparison:

http://www.el34world.com/Forum/index.php?topic=7957.0

With respect, Tubenit

[HotBluePlates]

... First the driver V4a. Put a 22µF cap across the 1KΩ cathode resistor. Then replace that 10K/3W plate resistor with a wire jumper.
...

Don't forget that massive voltage divider ahead of V4a... 1MΩ:47kΩ. That reduces the drive level into V4a by about 95%.

One trick you could try is replace the 1M and 47k temproarily with a 1M pot, with the wiper going to V4a's grid. Adjust for the desired level of reverb drive. You can measure the two halves of the pot at this setting and use the closest standard values in place of the existing 1M and 47k.

[nandrewjackson]

thanks HBP, Tubenit, and Sluckey,

I'm not gonna mess with it much more.  The layout on the board of that amp is kookie, ESPECIALLY around the reverb tube.  It's hard to get in there to trace it out, let alone start changing stuff.

I like the look of that "one tube reverb" from that link, Tubenit.

[frank57]

The JCA 2112rc is not an Astroverb circuit.
It is an Atomic 16 circuit with reverb.
The voltages in the preamp however are similar to the Astroverb.
Look at the schematic for the jca 20 h head that will take care of the preamp.
Not sure what they did for the reverb section.

The reverb definitely needs tweaking it's barely there.
The ones I tried had a nasty hiss if you turned the master and gain up together even before halfway.
Or tried to play clean with the master high gain around 2 or 3 does yours have the same problem?

[nandrewjackson]

Yeah, it has a lot of noise at high master volume.  It's not overwhelming, but it's annoying nonetheless.

I can't really complain too much about it.  I couldn't build that amp for what it cost, and it has good tone aside from the hiss.  I'm sure it could be upgraded with better caps and a choke. Maybe I'll do that someday.

[frank57]

It drove me nuts because it was showing up before halfway.
With the gain and master together around 4 or so.
Otherwise it was a good rock sound.
I wonder if it's coming from the artificial center tap resistors?
Maybe dc power p would solve it?

[nandrewjackson]

hmm,

Mine's not that bad.  With master volume at 8, 9, or 10 is the worst.  If I turn up Gain to 7 or 8 and master volume up to 5 or 6 it's not too bad.

It definitely needs a grounding upgrade.  I had mine apart for the reverb circuit tweaking, and didn't have the back panel screwed to the chassis. (the panel with the speaker outputs) and had the amp facedown on the floor to bias it so the panel was just laying in it's position w/o screws.  It was making the WORST noises ever and I thought I had messed something up while tweaking on it.

It turns out that simply screwing in about 3 of the 6 or so screws for that back panel made the ground better and that noise went away. 

I need to really go through it and upgrade the grounding, caps, add a choke, and maybe check out that filament heater circuit, but it's not really that big of a deal right now to me.

I can't play loud here in my apartment where the amp is most of the time, and when I'm jamming with a band you can't hear the noise over everything else, so why bother right now.

I'll just remember to NEVER try to record with it as the amp is now.

[frank57]

It seems to get worse over time.I remember the demo in the store was ok not too bad.
Then when I went back a few months later a ton of very audible hiss at about halfway.
Wonder if the chassis layout is causing something to overheat maybe?Or lousy B plus resistors causing more hiss?