HOLDEN/WASP

[TIMBO]

Hi guys, Just picked this up this arvo.

Holden were an amp builder in NZ (New Zealand) and in the early 70's jumped the ditch to sell then here. Some parts were made in NZ and some here in OZ.

These were the MUSLE amps of the 70's and there claim to fame is Angus playing them.
http://www.marshallforum.com/other-amps/60697-anguss-wasp-amp.html

This one is a baby compared to others with a pair of 6550's in the engine room.
Here is a bit more info..........
http://www.ozvalveamps.org/holden.htm
I can't say if the 6550's are a standard power tube but this one has got them.

[tubeswell]

Used to see those everywhere in NZ in the '70s

[TIMBO]

Hey tubeswell, Unfortunately these amp have had there shear of problems. Do you have any experience with them.Thanks

[tubeswell]

What sort of problems are you having?

[TIMBO]

Hey tubeswell, The preamp PCB are badly mounted and has the tubes horizontal and set way back in the cab. So I don't know if this is causing any overheating problems. The reports say that the preamp PCB has some warping that may crack the traces.

[tubeswell]

Rebuild a new PCB, or convert to PTP or eyelet/turret/tag board layout

[PRR]

I don't see a heat problem. Looks cooler in there than most chassis. Power bottles set where the heat rolls right out, and far apart. (Room for four power-jugs, still cool, but I think PT and OT would be bigger in a 4 big bottle amp; you see such a beast on that page you linked.)

You may however have a heat problem. Some early low-price PCB stuff really did not stand up in ANY tube application. Most of the Dynaco power amps have PCBs like toast, or a new-made replacement.

It is obviously in the Fender/Marshall family, Bassman derived. Pull the chassis. Fix what's broke. If PCB is toast, get a turret-board for a Bassman, cut where preamp/poweramp join, duplicate the values found on the toast-board.

[TIMBO]

Thanks PRR, There is some room for improvements. I haven't pulled the chassis yet to get an idea of its condition.

When the guy went to give me a demo it crackled a lot and did not sound too good.

[TIMBO]

Hi guys, This one is pretty neat and tidy.
Nothing stands out as a problem.
Some minor fixing is needed on the chassis and rust on the bell ends.
The preamp PCB is what has been a possible problem, but this one looks fine.
There is only two bolts holding the PCB mounted sockets against the chassis.
The Ecaps measure over marked value and also maybe close to the voltage rating, so these will be replaced and while the PCBs are flipped some spaces as well extra bolts will be added. Also I will check the traces on the B+ to see there condition are as well as the others.

Power tube sockets are very loose.
Wiper tag on treble pot loose and pot different to others.
It has a fixed, fixed bias. I'm thinking of making it a more adjustable or even going to a cathode bias.
I think it is a 100w job, and although the OT is huge it may pay to drop the output a bit. Thanks

[TIMBO]

Hi guys, ordered new Ecaps from Doug and this will be the first thing on my list to do before firing up.

I have asked the guys over here to put their 2 bobs worth in on things to do as I need to unsolder a couple wires to flip the PCBs.
The new Ecaps are a higher voltage as some would be borderline on there rating.
Compared to other amps (not that this may be apples for apples) but the filtering values seem a bit low for this amp????
Maybe the preamp cathode bypass are a bit iffy.
The two 560pf caps measured wrong compared to there mark value so some suggestions there.....

[PRR]

> RCA spec sheet shows 100w output in PP class A1.

Typo. Look at GE sheet.

[TIMBO]

Hi guys, I've mulling over the schem and looking for ideas. Its pretty much a JCM 800 with a twist.

Seeing that it has two channels that are mixed at V2 this would make it easy to voice each channel at V1.

I would like to make one channel for BLUES and one for ROCK, then hopefully jumper then for..........

While we are at it, the cathode follower off the PI do I leave it as is. Thanks

[TIMBO]

Hi guys, The input to the minor mods have been great. I have rebuilt the power board with .7w metal film resistors and 630v polypropylene caps and 500v F&T Ecaps.
The bias is now adjustable, with test points and external adjustment.
The speaker jacks also moved to shorten wires and accommodate the bias test points. 

[tubeswell]

[popcornmode] Keep up the good work [/popcornmode]

[DummyLoad]

looks really good. from what it was to now.


when are you going to fire it up?    


--pete

[DummyLoad]

i believe that this is what your PS looks like and only confirmed your schematic. the two taps marked red and black are what were throwing me. the black is the CT, yellows are the winding ends. 

i traced the attached schema off the pic of holden slave and pic is attached also, only yours is missing C2 & C3 (see attached) since your wasp is a 50W?

i don't think you need any resistors across caps C1 & C4 except for maybe quicker discharge, but not as balance resistors. the center-tap holds the cap voltage at B+ midpoint.

--pete

[TIMBO]

Thanks Pete, Your are brain saver, this was starting to do my head in 

Guys, I'll keep all the associated topics in one thread as Darryl suggested, it was just that these amps have some quirks that to me seem to be out of the ordinary circuits.

I did fire up this amp to get some voltage readings and when switched OFF all the Ecaps were charged. I haven't experienced a jolt from a 400v cap ,so I REALLY don't want one that could be 700v.
The indicator lamp on the B+ is not working so if this could bleed down the caps after the standby is turned off, it didn't do it. 

[sluckey]

Which bulb?

[TIMBO]

This one.

[sluckey]

I've seen the bulb on the previous schematic in another thread. I want to know what type bulb, incandescent, neon, etc., and which number, ie, 47, 49, 387, NE-2, etc. Trying to understand why it did not bleed the power supply.

[TIMBO]

Hi Sluckey, It looks like an incandescent and lights with a 9v DC battery. The 470k dropping resistor looks to be dead as I don't get a reading across it. Its a bit broken 

[TIMBO]

Hi guys, Got her all back together and plugged her in.

These are the power tubes...............
http://www.drtube.com/datasheets/6550a-ge1972.pdf
With no tubes in, at the top of the cap stack I get 717v and in the middle 375v.

The bias supply as per Sluckey's suggestion I get a range of -37 to -49v

The reservoir cap does not de-energise by itself, but all filter caps downstream of the standby sw. will slowly de-energise via the indicator lamp connected to the standby sw. This is a slow process and leaves about 60v left in the cans, so can a bleed resistor can be added here???

The reservoir cap (both 100uf cans)will de-energise with a clip between the + of the bottom cap and ground, will this be ok to do. Thanks 

[TIMBO]

Sluckey, On a closer look, the POWER lamp (red) is a neon and is working fine. The standby lamp (green) is incandescent, I tested it with a 9v dc battery and its all good(outer plastic casing is broken) but does not want to light with power on (approx.350v dc), it has continuity to ground across the bulb so it drains the caps very slowly.

[sluckey]

The bias supply as per Sluckey's suggestion I get a range of -37 to -49v

Just for info, my Sunn amp with 6550 has a bias supply that goes up to -60V. Been a while since I looked at it but I believe the bias is set approx. -55V. I would decrease the onboard range resistor to 680Ω or even 470Ω to see if you can increase that bias voltage a bit.

The reservoir cap does not de-energise by itself, but all filter caps downstream of the standby sw. will slowly de-energise via the indicator lamp connected to the standby sw. This is a slow process and leaves about 60v left in the cans, so can a bleed resistor can be added here???

I would just put a 220K resistor across each of the series caps. This will drain the entire B+ rail and you won't have to rely on the standby bulb to drain caps.

The standby lamp (green) is incandescent, I tested it with a 9v dc battery and its all good(outer plastic casing is broken) but does not want to light with power on (approx.350v dc)

Well, you have a 470K resistor in series with the lamp, so the absolute most current that could flow thru the lamp would be 350V/470KΩ = .7mA. That does not seem to be enough current to cause the filament to glow bright enough to be visible. I think you will need a smaller resistor or better yet, use a neon lamp. (Just for reference, a #47 lamp like Fender uses requires 6.3V @ 150mA.

That standby lamp looks to be an inappropriate replacement to me. I bet there was a neon there originally. Look at the pic of the similar slave unit posted by Dummyload. Power and Standby lamps look identical.

EDIT... The schematic I found does indeed show a neon for the standby lamp. There is no external current limit resistor shown so I assume there is a resistor inside the lamp base, like the power indicator lamp.

[TIMBO]

Thanks Sluckey, Found these at a local store http://www.altronics.com.au/p/s4016-red-220v-panel-mount-neon/

They have a built in resistor and close to our 240v power supply.
They have AC marked on them, so am I able to use them on the DC B+ supply as per schem. Will i need to add another dropping resistor because of the higher voltage. Thanks

[TIMBO]

Hi guys, Finally got up enough courage to put in the tubes and flick the switch. All good, with some crappy sound.

The preamp volume pot is stuffed, cuts out when turning.
I have not touched the preamp as I wanted to see what the mod sounded like and its not great so far.
I measured all components on the preamp board and nothing stood out as being faulty.
I can easily pull the preamp board and replace all components and see what that reveals.

I'm yet to find out what V4 should be, 12AU7/12AT7.

Voltages on sheet 6

[sluckey]

They have AC marked on them, so am I able to use them on the DC B+ supply as per schem. Will i need to add another dropping resistor because of the higher voltage.

The AC rating just means the lamp will work directly across 220VAC (or 120VAC) without any additional resistor.

That will work fine on B+. If the internal resistor is about 220K I'd try it without an external resistor. If the internal is only about 100K I'd use an external 220K to 470K.

[sluckey]

Quoted from http://www.ozvalveamps.org/holden.htm

The other oddity with all Holdens (including oz WASPS) is that the phase splitter uses one valve (12AT7 from memory) and then each phase drives half a 12AU7 which in turn drives the output stage.

[TIMBO]

Thanks Sluckey, After some tweaking of the bias circuit, I been able to get a good setting for the bias.

With the circuit as is I had a range of-39 to -47 @ -39 I get 38ma @684v approx. 26w
I paralleled the 10k resistor off the bottom of the adjustment pot (didn't want to pull board to change range resistor) and the range changed to -30 to-44 I now have it set @ -36 with cathode of 51mAs and plate of 684v output of approx. 35w/tube.
Will changing the 10k resistor the 4.7k be a problem as I did not want the pull circuit board??

With this new setting the amp is sounding great (haven't changed the PI/CF tubes).
So this has given me an idea of the added triode to the circuit. so far so good. Thanks

[sluckey]

Will changing the 10k resistor the 4.7k be a problem as I did not want the pull circuit board??

That's fine.

[TIMBO]

Hey Sluckey, I'm finding it hard to understand the bias for this amp.
The original preset bias was set at -54, so this was a ballpark voltage to aim for, for the adjustable circuit and with your suggestion for resistor value was close at -47.
This is where it looses me, with the bias pot at the max voltage of -47 the cathode was 14.3mA, so me thinking that I need to increase the bias above -47 and heading towards -54 was going to raise the cathode to a maximum output, BUT
Instead I have had to go backwards to -36 to achieve a reasonable output, SO less IS more.   

[shooter]

This is where it looses me, 

I was hung up for awhile on that, till the wise here used an analogy about brakes n gas pedals, The more neg the grid, the more it can resist the electrons coming off the cathode, slowing everything down, the closer to Zero the less resistance, allowing more electrons up n out the tube. I'm trying now to wrap my pea brain around when it goes Pos. (PS, I probably still have it wrong!!)

[DummyLoad]

Hey Sluckey, I'm finding it hard to understand the bias for this amp.
The original preset bias was set at -54, so this was a ballpark voltage to aim for, for the adjustable circuit and with your suggestion for resistor value was close at -47.
This is where it looses me, with the bias pot at the max voltage of -47 the cathode was 14.3mA, so me thinking that I need to increase the bias above -47 and heading towards -54 was going to raise the cathode to a maximum output, BUT
Instead I have had to go backwards to -36 to achieve a reasonable output, SO less IS more. 

as applied to the grid, the GREATER the value of the negative voltage causes LESS current to flow: the LESSER the value of the negative voltage causes MORE current to flow.


working with negative numbers the absolute value of -47 is 47: the absolute value of -54 is 54; so then 54 is greater than 47. -36V at the grids will cause MORE current to flow than -47V


--pete

[TIMBO]

Thanks guys, Hey Pete, its more the fact that it was originally set at -54 

[tubeswell]

Methinks it would have been set at -54 to be a nice safe bias voltage no-matter-what brand of 6550s you chuck in, or something to do with that way of thinking

[2deaf]

Methinks it would have been set at -54 to be a nice safe bias voltage no-matter-what brand of 6550s you chuck in, or something to do with that way of thinking

700V with -44V bias took out two sets of those GE special design 6550-A's along with a very expensive power supply with computer grade filter cap.'s circa 1978 in my 300PS.

You might want to consider -47V or even more negative.

[DummyLoad]

Methinks it would have been set at -54 to be a nice safe bias voltage no-matter-what brand of 6550s you chuck in, or something to do with that way of thinking

700V with -44V bias took out two sets of those GE special design 6550-A's along with a very expensive power supply with computer grade filter cap.'s circa 1978 in my 300PS.

You might want to consider -47V or even more negative.

i concur. -50V is safer. this is AB1 deep into B so ya, -54 was a safety. 35mA/6550 would be about 60% of 42W @ 700V B+


--pete

[TIMBO]

Hey guys, This is where I'm getting confused.........
With the bias circuit at max -volts of -47 my cathode reading was 14.3mv and 701 on the plates. My calculation of 701x .014=9.8w so if the minimum of -39 gets 45.3mv and plate of 692v the output of 31w.
I did not check what the cathode reading before I changed the circuit but my thinking is -54 and the plate was 718v would not have had a output at all 

[TIMBO]

OK, While we're trying to nut that out, its on to rebuilding the preamp board. This was needed as removing the fish caps most crumbled.

I've removed the pots and jacks from the front panel, its a bitza
I notice the TS was non responsive and a bit of tweaking is needed, Thouhgts.
The input jacks are not very exciting and have 47k, again thoughts.

[Ritchie200]

Timbo,

I don't now anything about the current crop of 6550's, but 700v+ on the plates is really humping.  I can tell you I went through countless versions of current production KT88's to find some that would live north of 600.  Just thinking out loud and my humble opinion: Living in 220-240v land, you probably do not have another tranny tap option.  However, I think your tubes would thank you and making for a reliable gig amp - it sure would be nice to knock about a 100v off those plates.  Any thoughts?

Jim

[TIMBO]

Hey Jim, There is no extra taps on the PT, But rewiring the rect/voltage doubler could drop volts down to about 350v.

[tubeswell]

... It sure would be nice to knock about a 100v off those plates.  Any thoughts?

Jim

dropping resistor in a CRC filter between the rectifier and the plate supply?

[PRR]

> cathode of 51mAs and plate of 684v output of approx. 35w/tube.

Are you confusing Idle Power with Maximum power?

Don't.

In a fix-bias amp, the idle current has "nothing" to do with maximum power.

Think of a car. The engine is good for 200 horsepower. When you don't need power, the throttle falls back on an Idle Screw that holds the engine at, say, 700RPM.

Will changing the idle speed to 600 or 900RPM change the maximum 200 horsepower? No.

And if you idle a lot, a fast idle increases engine wear. In your guitar amp, a hot idle increases tube wear.

Yes, if you diddle the engine idle down to 400RPM, you may get a hesitation when you step on the pedal slowly. In amps, a too-cold idle gives increased distortion at very low levels. It is a compromise. But you didn't drag a hundred-Watt amp out to play at One-Watt levels. In stage-amp duty, the benefits of a cooler idle may out-weigh any low-low-level roughness.

Many older stage-amps were factory-biased VERY cold. 14mA is not unreasonable. While you might hear some roughness played very-soft, when you LEAN on it it cleans-up. And as long as the drummer is awake, you are going to be playing in the higher-power zone.

30mA-40mA seems ample for 6550/KT88 at 684V. 20mA probably not wrong.

[2deaf]

One hundred watts?  I'll betcha those 6550-A's at 684V will put out 175W.  You know, I could show you how to measure that output (just kidding -- see the How do you Measure Output thread that just exploded).

[TIMBO]

Thanks guys, All the new bits are in and the amp is back together and sounding great.
The two channels are fairly typical marshall sound one being brite and twangy and the other dark and fuzzy.

As for the output and bias at 40mAs and everything at max I would say 40watts max sound wise.
I don't know if adding the MV and PPIMV has reduced the output compared to the original.

The tone stack I have changed to a typical marshall circuit.

Swapped the PI/CF around and added a 12au7 but this didn't sound much different.

[TIMBO]

Hi guys, After rewiring the preamp controls (new pots and jacks) it all worked well with No issues, SO I thought. The presence control did not seam to be to responsive and on a closer look, realised I had not connected the pot/resistor to ground, well that made a difference and the reason I missed it was cause I didn't trace that part of the circuit.

Now that the amp is sounding much better it's onto the fun bitz, I hope.

The first choice of OD was a Dumble style as I have an extra hole in the chassis for a 9 pin.

So the obvious think to do is steal some of T ideas and go from there.

A quick knockup of a breadboard and we're good to go.
I've started with the basic copy of T's D'Mars OD (no TS)and straight off the bat we have OD goodness.
I also have not added any SMOOTHING caps, this I will tweak in the process.
Stay tuned............   

[TIMBO]

Hi guys, Adding the OD was well worth it.

I thought I would do another circuit before I commit to the PCB.
The dumble circuit that T has based his creations on is what I have decided to do, although a couple changes have been done.
I'm finding that this circuit has heaps of BALLS and really screams MARSHALL.
I have placed it before the PI, this has the luxury of the two channels connecting to the OD.
The OD is relay switched ON/OFF.
The two inputs can be A/B to have some more options.

[TIMBO]

Hi guys, Things are going great. Although I MUFFED the relay circuit and a couple of other bits, so a new board was etched and I got it right this time. 

The OD is sounding pretty smooth and works well with both preamps and the LEVEL control can be set so that the clean and OD are similar volume when switched.

Some more tweaking needs to be done.

[TIMBO]

Hi guys, Cleaned up the box and its lookin good.

[TIMBO]

Hi guys, Having a wee problem.
The relay switching isn't doing its thing.
I have used this circuit with not problems but this one the relay is not disengaging when the switch is off.
When the relay is off the LED is on and there is 18v across the switch so I upped the dropping resistor to 820r thinking that this would reduce the current flow thus disengaging the relay as it should but that didn't work.