PA 300 Conversion

[Toxophilite]

HI all
Has anyone ever used a 6SL7 as a Phase inverter in an AB763 deluxe reverb Build?
I'm wondering because one of my earlier conversions (A Electrohome PA 300 which I've had for a few years ) has one in that role.
As I'm really enjoying my the AB763lite I just built I thought I'd make another
(The platform for my AB763lite is a very similiar Electrohome amp except it had a 12AT7 PI)

What's better than one deluxe?.  Two of course!

The 6SL7 does share some properties with the 12AT7 but it's also different

I'm reading through 'Understanding guitar amplifiers' (which is a cool book thanks!)

But I'm not exactly sure if changing the power amp section to be a higher power(via higher voltages and a different circuit) will mess with the PI circuit which frankly I'm pretty sure I currently don't have the knowledge to mess with. If I kept it how it is I'd likely have to make sure it gets similiar voltages as original

I know the Deluxe AB763 runs on a lot higher voltages than the original PA300 but my AB763lite build has the same PT and it's doing fine (just used it in a recording session friday eve..NICE!)

Here, once again, is the PA300 schematic

[stratele52]

IMO Leo fender choose the best tube for his amp .

How can you tell it is the PI 6SL7 tube in your electrohome PA made the tone you like ?

[jazbo8]

The EH PA-300 has a paraphase PI and the DR has a LTP PI, so their overdrive characteristics are different, but there is no reason why the 6SL7 wouldn't work as a LTP PI. Also check out the Gibson GA-6, it is closer to the EH PA-300's circuit topology.

[sluckey]

I have an amp with a 6SL7 split load phase inverter. The preamp also uses a 6SL7 with an AB763 tone stack. The power amp is cathode biased 6L6s. It's not quite what you're asking for but it may give you some ideas. The sound is very similar to the blackface sound but a little warmer. Don't know how much of the sound difference is due to the different tubes, different PI, or different PA. Probably a combination of all.

Anyhow, an AB763 preamp using octals feeding an octal split load PI feeding a cathode biased PA is a good sound.

Here's my amp. Everything indicates I'm using 6SN7s but they have been replaced with 6SL7s.

      http://home.comcast.net/~seluckey/amps/rocky/rocky.htm

[sluckey]

Oh, and here's another amp using a 6SL7 PI. This is a paraphase PI like that shown in your PA-300 schematic. This is straying farther from your Deluxe AB763 idea, but I'm just throwing some more info at you. Do you really want another AB763?

This little Ampeg is all octals and really sounds good. This is my radio conversion. Here's the schematic...

      http://home.comcast.net/~seluckey/amps/RCA/Ampeg_J12B.pdf

[PRR]

> Leo fender choose the best tube for his amp

With a keen eye on the price. By the 1950s, minis were considerably cheaper than Octals. Unless there was a compellingly good Octal (mostly not), he'd have found a happy mini.

> not exactly sure if changing the power amp section to be a higher power (via higher voltages and a different circuit) will mess with the PI

In general: scale the supply voltages the same amount on *both* driver and output, it will work.

A healthy driver can deliver a peak signal of 20% of its supply voltage.

The usual Audio Power tubes, in usual bias, need a peak grid swing up-to about 10% of their _G2_ voltage. (Note that a self-bias class-A amp may only need drive of 6% of Vg2, as in this case; a radical change to deep fix bias needs more, but this is often not a problem.)

So to take that ElectroHome from 320Vp/280Vg2 to a 2*6L6 at 500Vp 450Vg2, a 1.6X change of G2, shift the 280V to 450V. (Basically, leave it at G2 supply, or not-much lower than G2 supply).

And then check driver tube heat! At 280V, the EH driver runs 128V 0.68mA per triode. 1.6X higher on both is 205V 1.1mA. Multiply for Watts, 0.223 Watts. What is rating on 6SL7? (I bet it is over a Watt, but _you_ should check as part of your learning.) Also verify the 205V and 1.1mA.

[Toxophilite]

Lot's of good information!
I like your Jukebox amp Sluckey! Nice!, That's a crazy looking rectifier
I couldn't open the other amp file sadly...time for a new computer

I'm not married to the idea of an exact AB763. As long as it sounds good.works well and has a similiar power handling ability.

> Leo fender choose the best tube for his amp

With a keen eye on the price. By the 1950s, minis were considerably cheaper than Octals. Unless there was a compellingly good Octal (mostly not), he'd have found a happy mini.

I concur!
In my humble opinion, without offending fender devotees, I imagine that a lot of Leo Fenders decisions were likely at least as much about economics and availabilty as they were about building the ultimate amplifier.
And I'm a big fan of fender amps

The AB763 I'm going for, is the deluxelite using 2 6V6s in the power supply.
So more like 415 at the grids of the  the 6V6s. So that should shift my numbers down some 1.48 X
189V .952ma . Is that correct? I'll check it vs the 6SL7 specs

I'm also contemplating leaving it how it is and reconfiguring how I originally did the preamp section, which I believe could be configured better.
It sounds nice it's just not very loud and is lacking in bottom.

It also has a 6SF5 (single high-mu triode) that's always been a problem with being microphonic (despite swapping tubes)
A hunt for an appropriate schematic(ampeg maybe??) and maybe I could try a 6SL7 in there too , giving myself an extrs gainstage)

Still knocking around ideas. I'm going to pull the chassis and see what I did originally..it's been a few years.

[Toxophilite]

Oh
I forgot to ask
Is there a definitive source for explaining a paraphrase phase inverter and/vs a long tailed pair? (which sounds like a species of bird...or monkey)
Or could someone offer a quick explanation?(assuming there is one)

Thanks!

[sluckey]

http://www.valvewizard.co.uk/paraphase.html

It's simply two cascading inverting gain stages.

[sluckey]

You may be interested in this adapter...

http://www.ebay.com/itm/6SN7-6SL7-to-12AU7-12AX7-tube-adapter-adaptor-socket-converter-brand-new-1-piece-/161188185246?pt=Vintage_Electronics_R2&hash=item258790849e

[Toxophilite]

Cool
i'll look that up. and do some reading.
I think I'd need the reverse of that adapter, or I could just install a 9 pin socket
To clarify things a little here's a couple pictures
Happily everything is nicely labelled on the top of the chassis
Alas as this was my first conversion a few year ago it's pretty messy and there's several things I'd do differently now.
I just noticed though that I mistakenly installed a 100vdc coupling cap after the first gain stage.oops

Currently it has two channels
Two inputs feeding into the two triodes of the 12AX7 then out into volume controls
Out and blending together with two 270K resistors then into the 6SF5
then out of that into afender tone stack with volume control(master volume)
into the PI

Across the front left to right it has 2 volume controls (one chicken head one small and round)
treble middle and bass
Above each other are two negative feedback controls, one of which is the original presence control, the other controls the entire negative feedback, and then the master volume/on/off switch.  I was big into adjusting negative feedback when I converted the amp!

[Toxophilite]

Here's a reverb rocket schematic with a 6SL7 PI
I'm not sure if it's a long tailed pair or a paraphrase because I'm still royally ignorant
There's no voltages listed though
and I just missed a Garnet P45B for dirt cheap this eve...grrr

[sluckey]

I think I'd need the reverse of that adapter, or I could just install a 9 pin socket

If you had that adapter you could pull the 12AT7 that's in your recent AB763 build. Then put the adapter in the empty 9 pin socket and finally plug in a 6SL7 or 6SN7. Now your AB763 has a 6SL7 PI, although it may not be biased exactly right. And that's what you originally asked about.

Here's a reverb rocket schematic with a 6SL7 PI. I'm not sure if it's a long tailed pair or a paraphrase

Did you read the valvewizard link I posted? See any similarities?

[Toxophilite]

I did have a look at the article, thanks very much
Some of the information currently goes over my head , though I am trying, it was late.
I was comparing those circuits and could see some strong similarities and yet definite differences, so I wasn't quite sure.

I was thinking of converting my PA300 into another AB763lite type amp (I like stereo for guitar)but with the components the PA300 chassis currently has (though I might swap the 6SF5)
One of which is the 6SL7 in the PI position

I wasn't considering putting an 6SL7 in my just finished AB763lit build, though that would be an interesting experiment as it would give me a preview.

From my limited knowledge and research I think the 6SL7 would do the job just as well as the 12AT7, I was just hoping to determine the necessary changes in the AB763 circuit to facilitate using the 6SL7 in the circuit. (I wish I could've opened the second schematic you sent me)

I

[sluckey]

I wish I could've opened the second schematic you sent me

It's a standard pdf made with Adobe Acrobat Pro 9. I don't understand why you can't open it. You were able to open the AB763 Deluxe Lite pdf which is the same file format created by the same program.

[John]

From my limited knowledge and research I think the 6SL7 would do the job just as well as the 12AT7

This is ONLY my opinion, but I think you'd find the octal a bit more "musical", or sweeter sounding than the 12at7. FWIW, I've used 6sl7s in my last 2 builds instead of the 12ax7, and really like 'em. I do use a humdinger (see Merlin). My 15 watter is a paralleled 6sl7 into a 5879 into another 6 for the PI, and then a pair of 5881 reissues for the power tubes. Fiddling the knobs, I can go from Knopfler to Gibbons. Except for the playing part. 

[Toxophilite]

Yes it all comes down to the player doesn't it..all gear considerations aside!
  Thanks for the insight.

I'm pretty good with computers generally other than the infrequent desire to toss it out the window, My internet Mac can't take the adobe reader update..too old
I just checked my other thread and saw that I need to put the pdf on a thumb drive and walk over to my musc computer which is a little more skookum but is NEVER hooked up to the internet.
That's what I did with the last one but I had forgotten that that's how I dealt with it
My apologies for the frustration

[sluckey]

Is it a problem with any pdf or is it just mine?

Rather than trying to open a pdf thru your browser, down and save the pdf to your hard drive. Then see if Acrobat can open it without using the browser plugin. Does that help?

[Ed_Chambley]

Is it a problem with any pdf or is it just mine?

Rather than trying to open a pdf thru your browser, down and save the pdf to your hard drive. Then see if Acrobat can open it without using the browser plugin. Does that help?

No problem either way.  Open in browser or save as and open in acrobat.  Already got it, but this reminded how nice your Visio files are.

[PRR]

> 6SF5 (single high-mu triode) that's always been a problem with being microphonic

6SF5 is half of an un-improved 12AX7. (Rather- the 6(S)F5 was improved and doubled-up to make the 12AX7.)

So you could make Octal-mini adapters to use 12AX7 or any of the AM radio 1st-audio triode-diode tubes.

[Toxophilite]

Just a problem with MY computer. not your PDF
I always assumed it was my aging computer.
As I said it will work on my recording computer (that one never gets hooked to the internet)
I'm sure it works on any recently purchased and updated computer

The purpose of the internet sometimes seems to be to fill landfills with otherwise perfectly good computers via 'updates' often of dubious nature...that and the dissemination of information and of course, 'adult' entertainment sites. 

Yes the 6SF5, I was using it as another gainstage but I could yank it's socket and put in a 9 pin socket and thus use ar 12AX7.    orrrr keep the octal socket, rewire it and use a 6SL7 in the same capacity

Also when I did this conversion a few years back I just rewired the signal path leaving everything else intact
I was checking the voltages and they're hugely low on the 12AX7 (70 and 100) and
I noticed the cathodes are tied together..hmmm
I think it could be setup a lot more efficiently, at least in the preamp section

Even if I decide to keep the power amp AS IS I think a gut and rebuild is required for the preamp and perhaps the power supply as well..or at least new electrolytics

[Toxophilite]

Hey Slucky I was able to open your ampeg Jet12 build on my other computer
Nice job with the schematic
Very easy to read
Thanks
I just got back to this and I was wondering if you could take pity on me and tell me if this is a paraphrase inverter
My guess would be it is as it looks very different from the long tailed pair in my deluxe and simiiar to the one in the article
However I'm a little dim and was hoping you'd let me off the hook
I'm going to read the article more too
Some of those ampeg amps look pretty cool, I've never played one though and have no idea what they sound like.
Though I know they have a good reputation

[sluckey]

See reply #4

[Toxophilite]

Right, my mistake
Thanks for the redirection

Hey what's the word on old can caps. I know they're decades past their shelf life and they can dry up
I don't feel my Digital multi meter can test them adequately
My instinct lately is just to replace them all under the chassis, certainly I'll replace the paper one as it gives me the willies.

What's the consensus?
I saw on your Rockola build Sluckey that you left an existing one in
Any probelms/worries with that

[sluckey]

I don't trust old electrolytics whether in a can or not. The only amp I have that I used the original can is my little Pee Wee Hammond. The can actually works good for now, but I'm skeptical that it will last long. No more than I turn it on though it will probably out last me. I would never reuse an old electrolytic in an amp that I had to depend on or was gonna give to someone.

I saw on your Rockola build Sluckey that you left an existing one in

Ha. That's just an illusion. Both of those cap cans were gutted and then stuffed with new caps. Way more trouble than it's worth, but I just wanted to see if I could do it.

[Toxophilite]

Cool
That was my thought too
I axed them on the AB763lite build as suggested
I'm going to build a whole new power supply for this PA 300 reconversion
Peace of mind and saving the nice old transformers is worth it
Especially if I go with a design that ups the voltage.

Admittedly I've been using the PA300 conversion for the last 4 years at shows all over
with most of the original can caps!

[Toxophilite]

I think I' have an idea of what to do with this build
But I thought I'd sound it out first

First I should ask, as I have NO experience with Ampeg amps
What sort of power does the Jet 12 put out or any of the Ampeg 2 6V6s,like the reverbrocket
It seems like it would be more in the range of 12-15 watts due to the lower voltages, Yes No?
Anywho
I was going to keep the amp two channels with a few differences
I was thinking of using the Jet12 power amp and 6SL7 phase inverter like the ones in Sluckeys JEt12 build
THen maybe use a 6SL7 in my old 6SF5 socket  so I have an extra gain stage
Channel 1 would be built around the 12AX7 ala the AB763 preamp
Channel 2 would use the 6SL7 more like the reverb rocket (I have to figure that out a bit but basically in to the 6SL7, out into volume and tone back in them out into the phase inverter)
I might use a master volume before the phase inverter as I still have one there(it's the amps on/off switch too)
I guess the other option would be to use the 6SL7 just like the reverb rocket and have a bias vary tremelo and have the 1st triode going straight into the PI which would make this channel a fair bit quieter I imagine..but I would have two different tremelos going...ooooo

As I have used this conversion often as my echo amp but like to send a little dry signal to it as well
I'll have a 2nd channel that has a little Eq if I want to take the edge off my Super line out
Also I'll have a couple of different preamps to play around with in recording settings

How does this sound? (not literally haha)
Any ideas or alterations

[sluckey]

Use your good computer to look at the schematic for my "'54 Rock-Ola conversion". I think you'll like the preamp.

[tubeswell]

Oh
I forgot to ask
Is there a definitive source for explaining a paraphrase phase inverter and/vs a long tailed pair? (which sounds like a species of bird...or monkey)
Or could someone offer a quick explanation?(assuming there is one)

Thanks!

Steve said it simplest. Some of Merlin's articles are summarised versions of what's in his book(s) on pre-amps. The books have reasonably detailed explanations of LTPs but neither book contains descriptions on paraphrase inverters as such.

A paraphase inverter is just two (normal, inverting) gain stages - one side taking its grid input signal from the plate of the other one (so that the signals at both plates end up being 180 degrees out-of-phase with each other). Because the two stages are cascading (i.e. one follows the other in the signal path), the gain from the second stage would be much greater than the gain from the 1st stage's plate (which would otherwise create unbalanced inputs to drive the output stage's grids), so the 2nd stage's grid signal is heavily attenuated with a voltage divider (where it draws its input signal from the plate of the 1st stage). There are sometimes also other measures that are typically taken with paraphrase PIs to attempt to balance the gain of the inverter, such as having the second stage's cathode unbypassed. However, because triodes are non-linear amplifying devices, and because Paraphase PIs rely solely on normal inverting gain stages, then when Paraphase PIs are overdriven, the gain goes out of balance more and more.

LTP inverters on the other hand, have a normal (inverting) gain stage as the 'input' (or '1st') stage of the inverter, but the input signal for the '2nd' (non-inverting) stage is taken from the cathode, which is tied to the cathode of the 1st stage (instead of from the grid of the 2nd stage, which is instead grounded-with-respect-to-AC with a large capacitor). The LTP gets its 'inversion' from the fact that the signal at the cathode of the 1st stage is 180 deg. out-of-phase with the plate of the 1st stage, and so that when the (unbypassed) cathode is wiggling, it also wiggles the second side's plate with the same phase (and so the plates of the two sides end up being 180 deg. out of phase with each other).

Now because the gain on the 2nd stage is reliant on the strength of the cathode-wiggle (and the cathode wiggle mainly comes from having the cathode resistor unbypassed), then the gain from both stages is usually out of balance, unless the tail resistor is sufficiently large enough to 'limit the gain' on the inverting stage. That's why in fender LTPs, which have a 6k8 or 10k tail resistor, the plate resistors are usually 82k for the inverting stage and 100k for the non-inverting stage. Whereas in other LTPs which have (say) a 47k tail, the plate resistors can both be 100k and the inverter more or less balances. However, by increasing the tail resistor, the plate-to-cathode voltage is ends up being reduced (assuming the HT voltage remains unchanged), so the tradeoff for balance is gain, and the more balanced you get the sides by increasing the tail, the less overall gain the inverter has.

[Toxophilite]

Neato
I almost think I understand a bunch of that, or at least how the two function differently.
Quite interesting

Sluckey, I was looking at the rock-ola schematic and I was curious as to why the first two stages seemed (to me) to be feeding one right into the other and then into the tone stack and then the inverter.
What was the resoning/theory behind that ?
 I'm quite curious as I'd be inclined to put a little volume/tone stack between the first two gain stages of the 6SL7,(cause that's what I know)
'
I guess for each channel I only have the one tube and i was goign to build each around the tube and each would have it's own EQ.

[sluckey]

Sluckey, I was looking at the rock-ola schematic and I was curious as to why the first two stages seemed (to me) to be feeding one right into the other and then into the tone stack and then the inverter.
What was the resoning/theory behind that ?

You're looking at the original jukebox schematic. That has a preamp gain stage direct coupled to a cathode follower. The cathode follower in needed because the remote volume control is connected to the amp with a 50 foot cable. (Jukebox in the corner, volume control behind the bar). You need a low impedance driver for that long cable and 10KΩ volume control. The low impedance circuit is also less susceptible to noise pickup. But that's not what I wanted you to see.

The schematic I thought you would be interested in is my conversion. It's on page 4.

[Toxophilite]

OOOOPS
I opened up the visio schematics and didn't notice there was two, the original and the conversion
THanks for pointing that out!
It makes much more sense now!

[Toxophilite]

I'm just making up my list of bits to buy and I have a couple questions for S.Luckey  and whoever wants to chime in
I'm contemplating two ideas:

1. Just building two different preamps (one octal rockolastyle (haha), one 12AX7 AB763)in front of the existing PA300 PI and poweramp with a rebuilt power supply that would like supply higher voltages (like the JET12b)
Would the likely higher voltages I'd be achieving with an enhanced power supply upset the existing PA300 power amp and PI ?
Should I keep them at the values on the schematic? I know I need to send higher voltages than the schematic currently is to the AB763 preamp but I could probably work it out
I was thinking this might be an easier fun experiment...maybe not though


2nd idea-  Using the Jet 12b power amp with the rockola preamp in front with the 6SL7
I was also going to use the power supply from the Jet as well as my build has a 5Y3
As mentioned previously the other channel will be an AB763 12AX7 preamp

-Would I be best off using the PI from the Jet12B build or from the rockola?
To me it seems like the Jet 12 would be more approriate as it feeds 6V6s rather than 6L6s on the rockola
I'm not sure if that matters

-Also are the components used for the 6SN7s in your rockola preamp the same ones you used when you switched it to 6SL7s ?
Does anything need to be changed?


-On the Jet build there's a couple of caps without voltage values
There on the cathodes of the 6SL7 PI (100) and the 6V6s (20)
Are these small (25) volt caps?


THanks ever so much for your indulgence and patience !

[sluckey]

Let me start with just some general guidelines that I follow when doing a conversion. The major cost of any amp is the power supply and power amp. Preamps are cheap and easy to adapt to a wide range of power supply voltages. So, keep the power supply and power amp mostly original because those expensive components were designed to work together. And since you want a 6SL7 PI and your donor has one, keep that also. You will have a basic power amp with NFB loop that you know all the components works well together.

That leaves only the preamp to graft onto your power amp. I've shown you the Jet and the Rockola first because they both use a 6SL7 as a PI. But both also use a 6SL7 for a preamp. These preamps only require one more tube. If you want a nice warm sounding preamp, then use the Jet circuit. As a bonus you'll have an extra triode that you can use for tremolo if you desire. Or,,, if you want a mid scooped AB763 sound, build the Rockola circuit. The AB763 will work with the lower power supply voltages, but will sound a little warmer and not so in your face like a Twin Reverb.

If you're having a hard time deciding which type of preamp to build, then build both of them if you have the room.

-Also are the components used for the 6SN7s in your rockola preamp the same ones you used when you switched it to 6SL7s ?
Does anything need to be changed?

All components are the same.

There on the cathodes of the 6SL7 PI (100) and the 6V6s (20)
Are these small (25) volt caps?

You can use 25V caps under the 6SL7s but use 50V under the 6V6s.

There is a discrepancy on my Jet schematic. I have two different values listed for B+ voltages. I don't know which are correct and it will be a while before I could verify. Won't make a difference though.

And as always, if you feel a need, you can increase your B+ voltages by using a GZ34 or solid state rectifier.

Hopefully this info will help you decide which direction to take with this project. My basic approach is to keep the power amp as is, rip out all the preamp stuff, and graft in the preamp of my choice.

[Toxophilite]

I like it!
Easy Peazy!
Rebuild the preamps and recap the power supply properly (it was done piecemeal before by yours truly)
Thanks again for taking the time

[Toxophilite]

Here's the beginning of my power supply in the bottom left hand corner
I can still move it around if need be
I put in a couple of terminal strips in front of the rectifier.
I didn't want to go right over top of the rectifier as that seemed a little problematic and I'd have to have all the caps pretty high in the air. Is that a big deal)

I put a few caps in loosely to show the general idea

Is it a okay place?

I guess another option might be a little aluminum cap barn on the outside of the chassis where the old can caps are now. That would certainly free up space

The original as you can see had a can cap right behind the 6SF5 (which is being changed to a 6SL7) and beside the 6V6s.

It seemed maybe better to have it all in a row in one spot.

Then maybe I have room for building of the octal preamp where that can cap was, or overtop of the 6SL7

I thinking of putting the 12AX7 preamp on two terminal strips right above the 12AX7 or in front of it, depending on where it fits best

I'm going to be yanking most of the pots and rearranging all that too

I obviously have limited space

Good ideas??

Any suggestions ??

[Toxophilite]

No opinions on the power supply placement?
Would I be better of with an exterior Barn ?
I di something similiar on my other build but there's was a little more room and I could put it up against the back of the chassis
In this one that's where the recifier is and I didn't think it was a good idea to put it atop the rectifier, aside from heat stuff I just thought it was a little awkward

Is this too close to the 12AX7 preamp?

[sluckey]

No opinions on the power supply placement?

That's fine.

[Toxophilite]

So I've mostly finished my reconversion
Everything works but there's hum with the master volume turned up, even with nothing plugged in and the volumes on the channels turned down.
If I remove the preamp tubes the hum will go away but the amp will hum with only one of the preamp tubes in, it doesn't matter which

Most of my voltages are good
The power amp and PI voltages are pretty close to the original PA300 schematic though the 'bias' voltage(not sure if this applies to this sort of bias setup) should be 18 vdc but measures 24 on my amp.

THe 12AX7 preamp has 184 at the plates, close to a deluxe and by using some dropping resistors I got the 6SL7 preamp plate voltages down to 127 vdc which is 27 volts higher than Sluckey has on his 'Rocky' amp but lower than the Ampeg J12 6SL7 preamp plate voltage.

I've also rerouted my preamp B+ supply wires to be lower and more out of the way
and fixed a few other little things as far as routing goes

Prior to reconversion I used this amp for many years and I'm pretty sure hum hasn't been a problem, however the previous preamp setup provided much less gain.

I also have a couple of questions about heaters that could be related:

-I had to replace the octal socket for the 6SL7 preamp tube as one of the pins broke off.
It's heaters connect to the 6SL7 PI and the power amp tubes, as I neglected to note how they hooked to the original I just followed them from the 6SL7 PI through the poweramp tubes into the preamp 6SL7 and hooked it up in the same way the PI is hooked up..I want to hook them up the same way right, everything I've read says so.

- The other thing is that the 12AX7 preamp has it's own heater supply
I left this in

- The heater lead dress is original to the amp and I left it that way as it hadn't proven a problem
before
However I noticed the power tube socket heaters are wired together opposite each other 2-7 7-2 rather than 2-2 7-7 Is that right?
Things I've read suggest it isn't

The picture attached shows the amp basically how it is now, escuse the alligator clips, I was trying different dropping resistors in the power supply

[sluckey]

Everything works but there's hum with the master volume turned up, even with nothing plugged in and the volumes on the channels turned down.
If I remove the preamp tubes the hum will go away but the amp will hum with only one of the preamp tubes in, it doesn't matter which

The hum is probably due to wiring layout.

However I noticed the power tube socket heaters are wired together opposite each other 2-7 7-2 rather than 2-2 7-7 Is that right? Things I've read suggest it isn't

I say it doesn't matter. You don't have hum with the MV turned down or the preamp tubes pulled, so there's your proof.

Need to see a schematic that shows your circuit, especially since you have parts of two or more amps blended together. Can't be very specific without one.

[Toxophilite]

Thanks Sluckey

From what I've read about layout and lead dress there's a few schools of thought
Some want the heaters in the air some want them next to the chassis, similiar with high voltage B+ lines.
keep the high woltage away from the grids (most impotantly) and the plates
if they do come near each other they should cross at right angles

-I tried to keep all my plate and grid leads away from the heaters as much as humanly possible
-all the grid lines are shielded as are the lines between stages, though the plate lines aren't(they're very short!)
-I tried to keep all the runs short regardless
- the leads to the tone stacks are thick multistrand wired with heavy coating
(In the photo they're tucked against the front of the chassis, I tried bringing them out a little but it didn't change anything)
- Though some of the power amp is grounded to the tube socket mounting rings(original) I star grounded the rest of the amp.
(I already tried isolating and moving the ground of the preamp sections but it didn't make a difference)
- The ground lines do run a little higgly piggly from the points they're grounding but I didn't think that was as crucial, they all end up in the same spot.This could be ignorance speaking, though I approached my previous build the same way in regards to grounding.

I tried to keep the B+ lines away from everything else and have since fixed up the lines to the preamps.
- Using my terminal strips I lined things up as they are on a fender amp.
In retrospect point to point might've been a better option as I'd end up with a lot less actual wiring.

Is my problem that actual placement of the terminal strips and components?

Also I wondered if my input into the 6SL7 preamp is too close to the PI and power amp

Any helpful suggestions in that regard?

As far as schematic goes, I haven't a good layout program, though I imagine I could use illustrator and perhaps I will.

I might try lowering the voltages to the preamp sections further as it's realtively easy

[Toxophilite]

Most of my voltages are good
The power amp and PI voltages are pretty close to the original PA300 schematic though the 'bias' voltage(not sure if this applies to this sort of bias setup) should be 18 vdc but measures 24 on my amp.
Is this a problem and I can't quite figure out how to adjust it on this format (PA300 power amp and PI)

As I don't yet have a franken schematic.
This amp uses the PA300 power amp and PI with 2 separate channels in front of it

One channel is a  AB763 using a 6SL7 (ala S.Luckey's "Rocky" but without the Raw control) and the other is a standard 12ax7 AB763 preamp. these are separated by 220 K resistors where they feed into a master volume control and then the grid of one side of the PI

My Brother in Law had his scope on it and measured about 8 milivolts of hum on the outputs (I think that's correct) He seemed to think that was negligable . I'll try in actual rehearsal and gig usage and see what I think.

[sluckey]

Your master volume pot also has an AC power switch on it, right? Try this... Disconnect the two AC wires from the switch and pull the two wires to the back of the chassis, near the convenience outlets. Twist them together with a wire nut or tape. Does the hum get better?

[ac427v]

I wonder if your MV is exaggerating low level hum. I put a 500K master just before the phase inverter in my one channel 763 in place of the 220k or 47k resister to ground. When it was set on 5 (normal volume) there was no hum. On 10 it was noticable whenever I stopped playing (but the screaming gain was incredible!) The amp is living elsewhere now or I would try floating the heater ground at 50 volts to see if I could improve it. Good luck with your brainteaser.

[Toxophilite]

Voila!
A franken schematic (S.Luckey I hope you don't mind the preamp pagerism, Illustrator is somewhat unwieldy for drawing, I won't be using it anywhere but here)

I'm going to try the AC power thing . The switch was never moved to the back as the cabinet it's in can't accomodate that, and it didn't didn't hum before. But I will still try it. It's easy!

I was also wondering that I currently don't have any resistor running to ground between the preamps and the phase inverter, Many amps seem to have this. I do notice a dramatic drop in the hum when the master volume is turned down 5-20%
Would 100-220K to ground be something to try at that point?

[sluckey]

Voila!

I knew you could do it. Doesn't that feel better now?

I'm going to try the AC power thing .

This is just a test. If it's not causing any hum then put it back.

Would 100-220K to ground be something to try at that point?

The MV pot is a resistor. However, that's a BIGGGG resistor. Usually 1M is about as big a grid resistor as you'll see on a cathode biased stage. Try a 220K or 470K or 1M from the wiper to ground and see what happens with the hum. Better yet, get a proper size MV pot.

[Toxophilite]

Well
The AC wires didn't seem to make a difference though I've since tucked them more into the corner of the chassis

As I've used up my electronics budget for the month and don't have a skookum AC switch around I put a 1 meg resitor from the wiper to ground..and I put a 220K resistor to ground prior to the MV
This seems to reduce the hum a fair bit
It could be my imagination but the amp seems to break up a little earlier

I was trying it without the MV pot hooked up, a 220 K resitor and straight in to the grid of the PI

That seemed to be okay too
If I was to do away with the MV (as I've discovered I barely ever use it) what would be the best configuration prior to the PI?

Thanks again

[Toxophilite]

I found a good power switch in defunct organ so I might bypass my MV as I don't really use it
Any suggestions as to the best resistor to ground prior if I'm leaving out the MV?
Should I just have a 220k to ground after the 2 220K resistors seperating the two channels and then straight into the grid of the PI? Do I need anything else in there?

Oh yes and I used the amp all night (3 sets)last Saturday at a gig and it worked a treat as my echo amp, no problems