Holden Wasp VBA 200 Repair

[amosmosip]

Hi All
I recently got given a Holden wasp VBA 200w head in pretty bad shape and am working on repairing it.
As far as safety goes I am an electrician and am aware of the voltages I'm working with here, but I am in no way an amp guy and have a lot to learn.
I have found so much useful information on this forum and have been able to work through the recap and repair of the power amp stage.
The next stage is the preamp circuit which has a faulty component (capacitor?) that I cannot identify, I was hoping someone may be familiar with this circuit and could give me some pointers. 
Sorry the only things I have figured out so far is that 1 leg joins to ground the other joins to the capacitor(C1) and resistor(R1) with the other leg of capacitor(C1) going out to the treble and bass pots of channel 1     

[mresistor]

Hi do you have a schematic you could post please.

[tubeswell]

The single Holden schematic in Doug’s library:

https://el34world.com/charts/Schematics/files/_miscellaneous/Holden_wasp.gif

(This might not be what you have on the bench. There were a series of different variations on the theme - some amps had quads of 6550s etc)

Some random information on Holden Amps here:
https://ozvalveamps.org/holden.html

[amosmosip]

Thanks guys
The one I have has the quad 6550's and the preamp has presence and eq for each channel I'll trace the circuit and draw out a schematic.
what software would you recommend for drawing a schematic I currently have ExpressSCH

[tubeswell]

The main thing you need to know if you've never worked on tubes before, is that they need a bias voltage between the grid and the cathode to prevent the tube going into thermal runaway. This is a relative negative voltage which is usually achieved by one of 2 main ways, namely:


1) Setting the cathode at ground potential and supplying the grid with a negative voltage (from a separately derived negative bias voltage supply circuit) - termed 'fixed bias'. Lots of amps output tubes are biased this way - including Holdens.


Or


2) Setting the grid at ground potential and the cathode at a positive voltage (by putting a resistor between the cathode and the ground - i.e., a cathode resistor) - termed 'cathode bias' - or less commonly referred to as 'auto bias' (most preamp tubes are biased this way, and in some amps, output tubes are biased this way).


(There's another less common way of biasing preamp tubes called 'grid leak bias' where the cathode is set at ground potential and a large resistance of several megaohms is placed between the grid and ground potential - but that won't be found in a Holden amp)


Helpful stuff on tubes here:  http://www.valvewizard.co.uk


Other than that, power supplies in tube guitar amps are relatively standard high voltage supplies

[tubeswell]

Thanks guys
The one I have has the quad 6550's and the preamp has presence and eq for each channel I'll trace the circuit and draw out a schematic.
what software would you recommend for drawing a schematic I currently have ExpressSCH

ExpressSCH is fine for drawing schematics


Another easy to use .sch programme (which I use a lot) is JSchem. It is freeware but is easy and simple to use and has most common tube symbols etc. .sch files created in this can easily be exported as .pdf or .png files. Found here: http://jschem.bplaced.net

[tubeswell]

Also note that there are other ways of achieving negative bias voltage on high power output tubes (like 6550s) that can be found in some higher power amps where the grid is driven by a low impedance buffer source designed to keep the output tube grid leak resistance low (to prevent the output tube grid from building up too much heat and starting to emit their own electrons - and thereby losing bias voltage - in a cyclic spiral of destruction). I suspect your amp may (or may not) have such an arrangement.

Or there could be some other thing they've done. (the attachment I think may be found in Radiotron designers handbook; http://www.tubebooks.org/books/rdh4.pdf, or the last version of the RCA tube receiving manual, which IIRC can be downloaded from Franks Tube Data site - the go-to place for tube datasheets https://frank.pocnet.net)

[tubeswell]

Yet another thing you need to be aware of with output tubes is that they need sufficient screen voltage (usually several hundred volts) in order to get the output tube to conduct. If there is no screen voltage, a tetrode (such as a 6V6 or a KT88) or a pentode (such as an EL34 or a 6550) won't conduct. If you find yourself in this situation, suspect a blown screen grid resistor (which is also an indicator that the tube in that socket as gone irretrievably bad, which could have been caused by thermal runaway caused by loss of control grid bias - resulting in the screen grid overheating and melting/deforming and thus shorting to one of the other electrodes etc, or simply caused by cumulative heat stress on an old tube over time - whatever).

[Rontone]

The next stage is the preamp circuit which has a faulty component (capacitor?) that I cannot identify, I was hoping someone may be familiar with this circuit and could give me some pointers.

Here is another picture I found on a forum of the same/similar PCB, looking at other Holden amps it seems they were using a lot of Philips/Mullard caps in them, the mustard type, yellow chicklets and the older bead type ceramics, even some dogbones turning up

Here are some pics of a Philips Mullard bead next to a later 70's ceramic square cap from my own pile, looks about the same size, with a bevelled edge, when split open they have a similar sized plate inside, but those types are only rated for 100V, even the larger sized ones...

[Rontone]

Another picture from the Talkbass forum

https://www.talkbass.com/threads/holden-wasp-club.442508/

It seems it is a ceramic cap of some value, coming off to ground from after the EQ slope resistor, before the tone stack caps for bass/mid

[Rontone]

Here is a little .sch template that should be pretty close