Voltage on pin 6L6 1 and 8

Marshall Amp Discussion

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sonofmickel
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Voltage on pin 6L6 1 and 8

Post by sonofmickel »

This is on a lowly Crate Blue Voodoo 120.
This is a friends amp.
I am getting voltage on only 1 output tube on the connected pins 1/8. All the other Output tubes have zero volts on pin 1/8, as it should be.
The other three 6L6 tubes are conducting, this one tube does not get hot at all.


Pins 3/4 read 458-463 volts on all tubes.
Pin 6 is getting 460v only on the # 2 tube, must the only place with a tie-in point, weird.
Pin 5 has correct negative bias voltage -35 on all tubes.
The power tubes connect through a separate mini-circuit board that connects to the main board through two press-fit 13 pin connectors.

I received the amp dead. A few of the pins had scorch marks, I cleaned them up, the amp then made sound.

Weird?
Junky circuit board?
Gut it and build a real JCM800 inside?
sonofmickel
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Re: Voltage on pin 6L6 1 and 8

Post by sonofmickel »

Sussed it out.
It(hopefully) was a bad ground on power tube 4.
I just bridged the socket ground to the tube 3 pin 1/8 ground and now it sounds much better and the #4 tube is conducting.

Seems fine after an hour?

Another thing, these amps seem to have waaaay too much gain. I released one of the 221k plate resistors from servitude and paralleled another 220 to make it 110K and that cleared it up a bit. Also reduced the gain on the third stage cathode(there are 5 preamp stages on the gain channel!), I have a 5k1 on there now and that makes some bit of difference(makes the tone much more vowely).

The tubes are really hot!
Scorching.
Maybe it's junk?
Short lived success?
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xtian
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Re: Voltage on pin 6L6 1 and 8

Post by xtian »

Now that you solved your issue, check the bias and adjust it properly.
I build and repair tube amps. http://amps.monkeymatic.com
sonofmickel
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Re: Voltage on pin 6L6 1 and 8

Post by sonofmickel »

Oh so sorry...
I changed some stuff....cathode 2 was re-bypased with the >68.
The 3rd plate was returned to to 221k.
2nd still(modified to 110k).
sonofmickel
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Re: Voltage on pin 6L6 1 and 8

Post by sonofmickel »

it sounds ok now but there is too much gain in this circuit and too much treble.

I want JCM 800 with the volume knob all the way up and a Fender Twin half way down. Yeah...right
sonofmickel
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13 pin socket source?

Post by sonofmickel »

Anyone have a place to get both male and female 13 pin straight sockets?
The parts are J13 and J14 on the Crate BV120H schematic(page 3 lower right).
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dorrisant
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Re: Voltage on pin 6L6 1 and 8

Post by dorrisant »

Can you post a picture of what you need? I probably have one. I have a couple of complete boards on the shelf.

Also, you can gut and rebuild...
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Stevem
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N

Post by Stevem »

Circuit board crap!
On all the power tubes you need to jump the socket pins with a lenght of wire back to the board traces or you the amp will be cutting out and or burning up on the guy in short order!
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Re: N

Post by bluesfendermanblues »

Stevem wrote:Circuit board crap!
On all the power tubes you need to jump the socket pins with a lenght of wire back to the board traces or you the amp will be cutting out and or burning up on the guy in short order!
??
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sonofmickel
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Re: Voltage on pin 6L6 1 and 8

Post by sonofmickel »

I'll get pics up soon.

The amp guts pic looks "dumbly" : )

I have thought about hard-wiring the ribbon cables, that would solve everything with a solid mechanical connection but may make maintenance difficult for the the next guy.

I posited the "gut-n-rebuild" but the guy is attached to the horrific tone and layout of his "first good amp".

The thing is he loves my tone = plexi-ish beaks-n-feet build with a few different distortion/overdrive boxes. The Crate is not even close, oh well.
Cameron
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Re: Voltage on pin 6L6 1 and 8

Post by Cameron »

Some of these had an auto bias circuit...so you could just put in new tubes and it would self adjust...so make sure that's not the one you have ..I had the schematic of it but I can't find it...if I do I will post it.up ..but check to see if 1&8 are going to ground...or to the auto bias circuit.

These amps really don't sound that bad ...it's very useable with the right 4x12 ....if you are a Dumble guy ..no..this is probably not the amp for you
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dorrisant
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Re: Voltage on pin 6L6 1 and 8

Post by dorrisant »

sonofmickel wrote:
The amp guts pic looks "dumbly" : )
Yes, it is a #124.

Seriously though, I have two complete boards, what parts do you need?
"Education is what you're left with after you have forgotten what you have learned" - Enzo
sonofmickel
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Re: Voltage on pin 6L6 1 and 8

Post by sonofmickel »

Pics:
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dorrisant
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Re: Voltage on pin 6L6 1 and 8

Post by dorrisant »

Here is a pic of one of the BV60 (not 120) boards that I have. The set does have the 13 pin headers that you are looking for. The board is missing a couple of items... the channel switch on the front panel, the IEC socket and the XLR out. If you want just the headers let me know. I can pull them and send just the headers (male and female) to you if that would work better.

Let me know.
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sonofmickel
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Dorrisant for the win!

Post by sonofmickel »

Thanks for the parts Dorrisant!
That's all it took to get this stain of an amp back on it's toe.
I think the original parts are under speced for the voltage/current going through a 100 watt amp. I did a few mods to the dirty channel preamp to lower the gain a bit, not much but there is a better usable range now. I do think the clean channel sounds pretty good(really loud though) especially as an 80's pedal platform.
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