This Thing is Way Too Loud!

[Michael Smith]

I bought this used around 15 to 20 years ago, and really haven't played it too much. Last fall I put in a fairly well matched set of output tubes and set the bias to around 64% of maximum. The high gain input has lots of crunch and volume even at low settings. Looking at the schematic, the high gain input has 4 preamp gain stages, while the low input has 3. The low gain input is more useable, but gets loud pretty quickly, but with less crunch. I actually had to roll the volume knob down on my Les Paul to the point where the tone was suffering, so instead I plugged the guitar into my Fulltone FullDrive 2 and turned the overdrive and volume down quite low. I was playing it through a 1 x 12 semi open back cabinet using the 8 ohm tap (100 Watt Eminence speaker). I can't imagine it thru a 4 x 10 cabinet on the 4 ohm tap. The guy I bought it from was selling the matching 4 x 12 Sovtek cabinet, but I passed on that.

I don't have any questions about this amp, just thought I'd post about it. When it eventually quits, it will probably be essentially unrepairable due to the Soviet components used.

 

[corliss1]

What? There's nothing in there that isn't repairable and readily available. I've worked on tons of these with no trouble finding parts.

 

[radiocaster]

I don't have any questions about this amp, just thought I'd post about it. When it eventually quits, it will probably be essentially unrepairable due to the Soviet components used.

No. I changed all the electrolytic caps, and those IC caps are not Soviet, neither are the crappy Mouser pots, I changed one or two. Those jacks are also not Soviet and are terrible.

I'm surprised the amp is still loud, by now those caps would have lost some capacitance, as mine did.

Mine is not too loud, I guess my speakers are not very efficient. The low gain in is great for pedals: fuzz, overdrive, whatever.

I modded it to give more distortion on the high gain out (although the mod is on the common part) so it's more of a metal/doomy sound, but it's very noisy. Didn't like the high gain channel stock, too bright.

 

[Michael Smith]

No. I changed all the electrolytic caps, and those IC caps are not Soviet, neither are the crappy Mouser pots, I changed one or two. Those jacks are also not Soviet and are terrible.

I'm surprised the amp is still loud, by now those caps would have lost some capacitance, as mine did.

Mine is not too loud, I guess my speakers are not very efficient. The low gain in is great for pedals: fuzz, overdrive, whatever.

I modded it to give more distortion on the high gain out (although the mod is on the common part) so it's more of a metal/doomy sound, but it's very noisy. Didn't like the high gain channel stock, too bright.

OK I was wondering about the IC caps. Whoever changed them did a good job in soldering them in. I will have to take a closer look at the pots the next time I have the chassis out of the cabinet. The power and output transformers are massive, although I could not find any identifying marks on them.

 

[Michael Smith]

What? There's nothing in there that isn't repairable and readily available. I've worked on tons of these with no trouble finding parts.

Well, I did find a schematic for it, although kind of blurry, it does show the values of the various caps and resistors. The values on the caps inside the amp itself are not recognizable to me, except for the IC ones.

 

[Michael Smith]

It's so loud that I was afraid I would blow out the 100Watt speaker at more than 25 % volume on the high gain input. It puts my Bassman 50 or Super Reverb to shame.

 

[Michael Smith]

Is this a clone of a Marshall circuit? If so which one?

 

[Swirling Snow]

The "way too loud" thing makes me wonder about the volume control. Next time you're in there, try to determine if it's linear. That would explain why it gets so loud so fast.

 

[Michael Smith]

I may hook it up to my 2 x 12 Super Twin Reverb cab and crank it up (with ear protection) and get out my decibel meter to see how loud it actually is. But my neighbors may file a complaint, lol.

 

[Michael Smith]

The "way too loud" thing makes me wonder about the volume control. Next time you're in there, try to determine if it's linear. That would explain why it gets so loud so fast.

I will try to see if there are any indications on the pots the next time I have it outside of the cabinet. It's kind of hard to re-install, getting the screws to line up.

 

[loudboy]

This is a JCM800 clone, right?

 

[Michael Smith]

This is a JCM800 clone, right?

That is what I would like to know. I don't know where I found the schematic online, but would like to compare.

 

[radiocaster]

OK I was wondering about the IC caps. Whoever changed them did a good job in soldering them in. I will have to take a closer look at the pots the next time I have the chassis out of the cabinet. The power and output transformers are massive, although I could not find any identifying marks on them.

The IC caps are from the factory. All the Western stuff in it is stock, like the pots and jacks. Your amp is stock.

This is a JCM800 clone, right?

There are similarities, but it is pretty different.

 

[Michael Smith]

The IC caps are from the factory. All the Western stuff in it is stock, like the pots and jacks. Your amp is stock.


There are similarities, but it is pretty different.

Thanks radiocaster. The most recent dates I could see on the capacitors were from 1995. This thing gets pretty hot with the rear metal grate on it. The cabinet is very small and the 6L6's put off a lot of heat. Since it resides in my studio, I plan on taking the rear panel off of it and running a small computer fan near the back to help keep it cool.

 

[Lynxtrap]

What does the master volume knob do?

 

[Michael Smith]

The "way too loud" thing makes me wonder about the volume control. Next time you're in there, try to determine if it's linear. That would explain why it gets so loud so fast.

Looking at the somewhat blurry schematic with a magnifying glass, it looks like both the volume and master volume pots are 1M Log. The treble pot is 500K Log, Bass 1M Log and midrange 25K Lin. The presence pot appears to be 5K Lin.

 

[Michael Smith]

What does the master volume knob do?

According to the schematic, the "first" volume pot is between the 2nd and 3rd gain stage of the preamp section. The tone stack and the master volume pot are after the 4th gain stage and just before the phase inverter. So the master volume pot controls the level of signal going to the 2 power tubes.

So if you want a lot of crunch but at an overall lower volume, you would turn the volume pot up and the master volume down, but it goes from quiet to loud very quickly.

 

[Lynxtrap]

According to the schematic, the "first" volume pot is between the 2nd and 3rd gain stage of the preamp section. The tone stack and the master volume pot are after the 4th gain stage and just before the phase inverter. So the master volume pot controls the level of signal going to the 2 power tubes.

So if you want a lot of crunch but at an overall lower volume, you would turn the volume pot up and the master volume down, but it goes from quiet to loud very quickly.

Seems like the problem might be the taper of the MV pot then.
It just seemed a bit strange to me that an amp with a master volume can be "too loud", unless you're trying to get overdrive from the power amp.

 

[Lynxtrap]

You could try something like a 250K Log pot as master volume.

 

[Michael Smith]

Seems like the problem might be the taper of the MV pot then.
It just seemed a bit strange to me that an amp with a master volume can be "too loud", unless you're trying to get overdrive from the power amp.

By "too loud" I mean that when the preamp volume pot is about halfway up, turning the MV pot up to even 10% it goes from quiet to very loud very quickly (when using the high gain input especially). I think you're right about the taper of the pots being "wrong".