Weird Heater Supply Problem - And other solutions!

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neikeel
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Re: Weird Heater Supply Problem - And other solutions!

Post by neikeel » Sun Oct 22, 2017 5:45 am

Yes, I agree

A lot of people leave those caps out, usually bedroom players.
Neil

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Tue Oct 24, 2017 11:55 am

Ok, in an effort to figure out why my JCM800 2203 is very dark sounding even with 6L6 output tubes (which I have always associated with being bright and with extended low range), I am going to list the differences between the standard JCM800 2203 specs than what I have cooking in my amp.
  • 6L6 Output Tubes
  • 525vdc loaded B+
  • Higher B+ in the preamp than spec'd due to different PT
  • Non Center Tapped HV and Filament - Commensurate mods to make it work
  • 1K 5 Watt screen grid resistors instead of 470 ohm 5 watt screen grids usually spec'd for a 6L6
  • Bugera 1990 Power and Output Transformers - This is a JCM900 4100 knock off amp
Everything else is exactly the same in the preamp through the phase inverter. Power section is only modified for a full wave bridge rectifier.

The amp is loud, VERY loud. But it does not have the typical JCM800 bright, rip your head off cut that I am expecting as my baseline before hot-rodding it.

The only thing that I am leaning towards is the cheap Chinese Bugera output transformer, spec'd for a JCM900 clone, is not the right OT for a 2203. How much of a difference would this make?

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Tue Oct 24, 2017 12:16 pm

How can an amp be JCM800 bright but dark at the same time? :scratch:

The OT can make a difference but I doubt it would make that much of a difference. I personally think of the OT as fine tuning. I don't think it can make or break an amp, unless it's horrendously bad!

To lower the voltages in the preamp you should have two 10K droppers between the screen node and PI node. Can't remember if you've done this or not.

Leave the 1K screen grid resistors in as you're extending the life of the valves with this instead of 470Ohm. You won't notice any tonal difference with this anyway unless you run the master high and drive the hell out of the power amp. Even then 470Ohm vs 1K won't make much difference.

The bright cap you installed, you say it didn't make any difference, are you running the gain on 10? If so then you won't notice any difference at all.

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neikeel
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Re: Weird Heater Supply Problem - And other solutions!

Post by neikeel » Tue Oct 24, 2017 12:51 pm

I am puzzled as none of the things that you have done would cause what you are hearing, unless the speaker cab is no good. Lowering preamp voltages will not brighten it up, but will help it break up earlier.

- it is probably the OT, assuming no wiring errors.
Neil

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Tue Oct 24, 2017 1:57 pm

Actually we have overlooked one thing. What guitar is it? What pickups? And are you plugged straight into the amp? Also what settings are you using?

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Tue Oct 24, 2017 4:18 pm


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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Tue Oct 24, 2017 4:22 pm

Ok, the other details.

I am using a 1x12 Jet City cab with a Eminence Wizard in it at 8 Ohms. I have also used a JCM800 1960a with Veteran 30s in it also at 8 Ohms.

The guitar is a maple necked strat with a Dimarzio Super Distortion in the bridge. Also used an Axis 20 with stock pickups in it.

No effects. Straight in. Treble at 10, presence at 10, Mids at 5 and bass at 5. All of the tone stack works great. Its just not cutting like I remember 2203/4s being. I had an 1982 2204 with EL34s in it that had plate voltages at 390vdc.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Tue Oct 24, 2017 4:48 pm

Not sure if I'm looking at it right but it looks like there isn't a 4.7K resistor bypassing the presence pot to ground? Without that the negative feedback circuit won't work correctly which would explain it.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Tue Oct 24, 2017 4:51 pm

Littlewyan wrote:
Tue Oct 24, 2017 4:48 pm
Not sure if I'm looking at it right but it looks like there isn't a 4.7K resistor bypassing the presence pot to ground? Without that the negative feedback circuit won't work correctly which would explain it.
I took it out on purpose. It was causing scratchiness. The pot does work and so does the negative feedback.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Tue Oct 24, 2017 4:58 pm

Ok that doesn't sound right. The pot will work but it's only grounded through the 0.1uF capacitor at the moment. So essentially the feedback network is only working on frequencies that can go through that capacitor, which would explain why the amp sounds so dark.

On the other hand it definitely shouldn't be scratchy with the resistor!

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Tue Oct 24, 2017 5:18 pm

Ok, I will give that part of the circuit another once over. I may try the 5K pot instead of the 25K with the resistor and all that stuff.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Tue Oct 24, 2017 5:23 pm

To be honest if you don't mind the scratchiness the 5K Pot circuit (on the older amps) has a much better range. HOWEVER, you might find it too bright set at midway. Simply because the 25K Pot with the extra resistor doesn't actually do much until you turn it past 5. Hope that makes sense.

Personally I prefer the older circuit but I have the newer one in my Master Volume Amp.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Tue Oct 24, 2017 5:28 pm

Littlewyan wrote:
Tue Oct 24, 2017 5:23 pm
To be honest if you don't mind the scratchiness the 5K Pot circuit (on the older amps) has a much better range. HOWEVER, you might find it too bright set at midway. Simply because the 25K Pot with the extra resistor doesn't actually do much until you turn it past 5. Hope that makes sense.

Personally I prefer the older circuit but I have the newer one in my Master Volume Amp.
I didn't care about the scratchiness either, but it didn't sound right with the resistor installed anyway. So I simply removed it. I will go back to it and try again. Ha! :palm:

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Tue Oct 24, 2017 5:35 pm

Yeah give it a go mate. Let us know :)

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Thu Oct 26, 2017 1:19 am

Gave it a go I did. Didn't make a difference.

So I then went all experimental on the amp and decided to swap capacitors around in order to brighten up the amp. It ended with me taking a .0022 cap for the phase inverter capacitor and it did brighten the amp up slightly. But it is still muffled sounding and congested.

I am 99% sure the output transformer is a severe bottleneck or something. I unsoldered the OT from the amp and measured primary resistance of 30 ohms.

Does that sound right?

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