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Author Topic: Peavey Heritage VTX ..uh oh..  (Read 29933 times)

Jack1962

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Re: Peavey Heritage VTX ..uh oh..
« Reply #15 on: July 11, 2010, 04:43:49 PM »
are you sure about that +15 volts on a tube grid JM

ohmy!!

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Re: Peavey Heritage VTX ..uh oh..
« Reply #16 on: July 11, 2010, 05:07:42 PM »
The resistors check out to the proper ohms as marked.  It's one of the transistors.  Q7 is bad.  I'll just go ahead and replace them all like you recommended, they're 25+ years old after all.

ohmy!!

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Re: Peavey Heritage VTX ..uh oh..
« Reply #17 on: July 11, 2010, 05:34:02 PM »
Also, in case anyone else needs these, there's a site  www.tubesandmore.com that has them with the original parts numbers.

just search SJ6465 and MPS 6531

Enzo

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Re: Peavey Heritage VTX ..uh oh..
« Reply #18 on: July 12, 2010, 02:24:31 AM »
+15v on the control grid is perfectly reasonable when the cathode sits at +90v.

Here, explore the power amp circuit.
http://www.schematicheaven.com/newamps/peavey_heritage_vtx.pdf

teemuk

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Re: Peavey Heritage VTX ..uh oh..
« Reply #19 on: July 12, 2010, 09:52:50 AM »
The Peavey VTX -series power amp's biasing circuit -related patent is U.S. #4,439,742.

http://www.google.com/patents/about?id=Tsc6AAAAEBAJ&dq=4,439,742

Jack1962

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Re: Peavey Heritage VTX ..uh oh..
« Reply #20 on: July 13, 2010, 11:21:42 AM »
no doubting your word guy's , however , evertube amp I have ever worked on cathode or fixed bias , had a negitive voltage on the pin 5 the grid.

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Jack1962

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Re: Peavey Heritage VTX ..uh oh..
« Reply #21 on: July 13, 2010, 11:25:36 AM »
just looked at the diagram , that's very interesting.

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Enzo

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Re: Peavey Heritage VTX ..uh oh..
« Reply #22 on: July 13, 2010, 09:06:08 PM »
Look at the Music Man amps, they are of similar circuitry.


When the cathode is grounded like most fixed bias amps, then the grid is indeed generally at negative voltage.   But cathode biased amps generally will not have a negatiove vokltage on the grid, because the cathode is made positive instead.

The whole point of bias is the relationship between the cathode and the grid.  What the voltages actually are is less important.   Having a tube grid at -25v and the cathode grounded is exactly the same as having the cathode at +25 and the grid grounded, as far as the tube is concerned.

SO having a cathode at +90 and a grid at +15, is the same to the tube as having the cathode grounded and the grid at -75v.   Either way the grid is 785v more negative than the cathode.   And in the case of the Heritage here, if you think about it, that is pretty cool bias.  These amps are working close to class B.


Obviously since the voltage across the tube plate to cathode is important, B+ would be different in each example.

phatt

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Re: Peavey Heritage VTX ..uh oh..
« Reply #23 on: July 14, 2010, 01:48:20 AM »
Hi Enzo Thanks for the explaination of the vastly complex.

But ya know One can't help but think Geezz that's about the most complex way around a simple problem? :o
Phil.

Enzo

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Re: Peavey Heritage VTX ..uh oh..
« Reply #24 on: July 14, 2010, 06:48:10 AM »
Oh poop.   It might have been even clearer if I had typed that the grid is 75v more negative than the cathode instead of 785v.  Oh well.

Well, when you are designing something it is only complex once.  Once it is done it is done.   Peavey had a number of amps using that basic power amp design, just as Music Man did.   I couldn't tell you who thought of it first.

I am no expert on this, but for the user, the design complexity is not an issue.  But out of it he gets a pretty efficient amp that is strong and won't really have any bias issues.   These darn things ar LOUD.

Hartley Peavey has written a number of white papers on various aspects of his product designs.  This is just the sort of thing he might write about, though I haven't looked.

Jack1962

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Re: Peavey Heritage VTX ..uh oh..
« Reply #25 on: July 14, 2010, 07:36:17 AM »
All I can say about the design of the bias circuit is it is rock solid and allows the end user to change tubes without rebiasing the amp , which most don't know how to or aren't capable of doing , which is a great marketing tool lol . 2 thumps up on this design . AS for the operation of the vacuum tube , cathode has to be more negative charged than plate , grid introduce's a small signal to produce a large signal at the plate, if you need more info read Jack Darr's book lmao

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ohmy!!

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Re: Peavey Heritage VTX ..uh oh..
« Reply #26 on: July 14, 2010, 02:31:45 PM »
The new transistors fixed it, other than the tubes which are all bad and sound god-awful.  Looks like I'll be ordering some tubes soon.

bry melvin

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Re: Peavey Heritage VTX ..uh oh..
« Reply #27 on: July 14, 2010, 03:28:10 PM »
The last tube sets I put in mine were chinese st shape "Chinese 6L6GC Small ST Shape"  from Danny's amp service in NJ  with his label

I THINK the ones antique radio supply (tubesandmore.com) are the same thing...39USD for the quad.

FWIW those sets were bought in 94  still working.



ohmy!!

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Re: Peavey Heritage VTX ..uh oh..
« Reply #28 on: July 14, 2010, 04:17:16 PM »
I was running some Sovtek 6L6 WXT+ tubes in there and they held up great until those transistors went out.  I may get another set of them.

Jack1962

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Re: Peavey Heritage VTX ..uh oh..
« Reply #29 on: July 16, 2010, 03:42:50 PM »
Ohmy you can order a set of Ruby 6L6's from musicians friend probably cheaper than anywhere else ($34 a pair I think) I have them in one of my amps and have had for a few years they seem to be pretty good tubes for rebranded tubes lol lol lol