Parts for Peavey VTM Bias Mod

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kiometalfan
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Parts for Peavey VTM Bias Mod

Post by kiometalfan » Sat Apr 16, 2016 10:45 am

Pretty soon I will be turning in my Peavey VTM 120 for a screen resistor and bias mod. My guitar tech told me that if I could get the parts beforehand, I'll save myself some significant money and the turn around time should be relatively short, so I've been trying to buy everything I need to do these two mods. So far I found where I would buy the resistors for the screen resistor mod. How many of these do I need? Four? Here's the link.

http://www.parts-express.com/470-ohm-5w ... e--015-470

Now I'm aiming to get a 15k limiting resistor and a 50KL pot for the bias mod, but I'm confused about exactly what kind of limiting resistor and 50KL pot I should get. Is there a specific wattage I should be looking for for the resistor? Or a particular size/type of pot? I'm totally new to this type of stuff and all my knowledge so far has been through reading, no hands on experience. If you know anything on this subject, feel free to post a link to the specific part I'd need, that would be greatly appreciated.

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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Sat Apr 16, 2016 8:48 pm

Did your guy give you a parts list?

15k resistor? ANy old resistor will do, say a half watt.

A pot of any convenient dimension is fine.

kiometalfan
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Re: Parts for Peavey VTM Bias Mod

Post by kiometalfan » Sat Apr 16, 2016 10:01 pm

No he didn't actually. He's a pretty busy guy. Okay well I found all those parts. I think I need one more resistor for the bias mod but I have no idea what the value on it should be. The diagram that the link I provided states that I need a 2k7 resistor? As far as changing the screen resistors in the amp, do I need to buy only one resistor or four?

I'm basically looking at this to figure out what I need.

https://www.flickr.com/photos/3rdof3/se ... 999774416/

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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Sun Apr 17, 2016 8:06 am

Four tubes, four screens, four resistors.

2k7 means the same thing as 2.7k, or 2700 ohms. it is really a better way of writing it. If I write 2.7k, the period dot can get lost and someone reads it as 27k. So 2k7 is not ambiguous. Likewise 2M2 is 2.2 Meg, and 4R7 is 4.7 ohms. You don't need those last two, but I threw them in as a lesson.

kiometalfan
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Re: Parts for Peavey VTM Bias Mod

Post by kiometalfan » Sun Apr 17, 2016 12:48 pm

Right on. Okay that makes sense, but is definitely confusing for a noobie when it isn't explained. I appreciate that. So I'm guessing I should get the half watt 2k7 resistor, correct?

Here's all the stuff I've picked out from Parts Express so far.

https://www.parts-express.com/15k-ohm-1 ... s--002-15k
https://www.parts-express.com/470-ohm-5 ... e--015-470
https://www.parts-express.com/parts-exp ... t--023-550
https://www.parts-express.com/27k-ohm-1 ... --002-2.7k

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Re: Parts for Peavey VTM Bias Mod

Post by vincenz » Thu Oct 08, 2020 2:08 am

____________________________________________________________________

Hi,

I am reactivating this topic as I just acquired a VTM 120 and I would like to make the bias adjustable with a potentiometer.
https://www.flickr.com/photos/3rdof3/se ... 289769037/


1. I easily understand the potentiometer installation/mod...

Image




2. ...but I don't understand the output tube mod below. What is the goal of that ? :

Image

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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Thu Oct 08, 2020 5:29 pm

You made the bias adjustable, presumably to measure tube current and adjust for it. How will you measure that current? Usually we add a 1 ohm resistor between power tube cathodes and ground. We then measure voltage drop across that. Your power tube board ground circuit goes through J7. Any chance your mod set includes adding a 1 ohm resistor at J7?

In any case, the stock board can only serve 6L6/5881 types, but the mod here allows 6550 or EL34 to be used.

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Re: Parts for Peavey VTM Bias Mod

Post by vincenz » Fri Oct 09, 2020 1:08 am

Thanks Enzo for that technical explanation.

I thought I could simply measure the plate current with my usual bias probe, then adjust it with the newly installed potentiometer (1)...
Am I wrong?

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Re: Parts for Peavey VTM Bias Mod

Post by vincenz » Mon Oct 12, 2020 7:30 am

Thanks for the answer.
Actually I thought I could measure plate current with a bias probe. Is the mod or the output board still needed in this case?
(and I wont use anything else than 6L6)

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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Mon Oct 12, 2020 1:00 pm

Oh you don't HAVE to use 1 ohm resistors, that is just the most common way they go about it. A socket bias probe works, and doesn't care about the amp. It is just in series with the tube.

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Re: Parts for Peavey VTM Bias Mod

Post by Faldoe » Fri Nov 13, 2020 11:24 am

I tried the bias mod as illustrated above and I'm not getting any MA reading/the power tubes don't seem to have any current. I didn't rewire the power tube sockets for EL34s, just the adjustable bias mod as shown in vincez's post.

Plate voltages are there.

I added a 1 ohm cathode resistor - I cut the PCB trace to one of the power tubes for Pin and and added one end of the resistor there, and the other back onto the ground pad it was connected to.

Any thoughts here on what is going on?

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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Fri Nov 13, 2020 8:29 pm

Look at the layout drawing, it includes the trace work. WHERE did you cut and WHERE did you wire a 1 ohms?

Note ALL the pins 8 are wired together, and the traces at one end go to a couple connectors, and the other end grounds a cap from the heater offset. Not to mention J7 in the center connects the board ground to chassis. Also teh fl;yback diodes from the plates use this ground trace.

SO if you break that trace somewhere you may be isolating parts of the circuit.

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Re: Parts for Peavey VTM Bias Mod

Post by Faldoe » Tue Nov 24, 2020 6:24 pm

Do I also need to isolate Pin 8 for all the tube sockets or just one? And do I have to do the "Bias modification" as posted earlier in this thread with the extra trace cuts or is that just to be able to use EL-34s?

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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Tue Nov 24, 2020 8:14 pm

ALL pin 8s must connect to ground, either directly or through a 1 ohm resistor. If you only want to monitor one tube, break the pin 8 connection and add a 1 ohm resistor there. If you want to individually monitor them, it will take four resistors. If you want one measurement for teh group of four tubes, then all pins 8 must be connected together and THEN a 1 ohm resistor to ground.

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Re: Parts for Peavey VTM Bias Mod

Post by Faldoe » Mon Dec 07, 2020 1:33 pm

Enzo wrote:
Tue Nov 24, 2020 8:14 pm
ALL pin 8s must connect to ground, either directly or through a 1 ohm resistor. If you only want to monitor one tube, break the pin 8 connection and add a 1 ohm resistor there. If you want to individually monitor them, it will take four resistors. If you want one measurement for teh group of four tubes, then all pins 8 must be connected together and THEN a 1 ohm resistor to ground.
Is there any reason to do one over the other? Will monitoring all of them improve the biasing?

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