How would you lay this tone circuit out? Weird Valco content

[Duncan]

Howdy.

I've got an old Harmony 430 (Valco-built, very similar circuit to the Gretsch 6162 - as similar as these amps could get), that I'm interested in tracing the circuit, ultimately to possibly clone it in a non-rat's nest way.

The schematics I've found for both the 430 and 6162 have the tone circuit shown like this:



I see how its applied inside the amp (components along a turret strip), but I'm curious how you guys would lay it out on a turret board.



Thanks in advance!

[tubenit]

Maybe something like this?         Check for errors!


I'd drill a hole in the turret board near "G" turret and near "T" turret and run a wire under the board to each of the pots.

With respect, Tubenit

[Duncan]

Here's my best guess?

From what I can tell, this is how it would be mapped out on a Hoffman board (DIYLC):

[Duncan]

Maybe something like this?         Check for errors!


I'd drill a hole in the turret board near "G" turret and near "T" turret and run a wire under the board to each of the pots.

With respect, Tubenit

Well that's a much more economical layout than what I had drawn haha.

Thank you!

[sluckey]

Here it is on a sluckey board...

[Duncan]

Here it is on a sluckey board...

Well that looks pretty slick!

From the schematic, it looks like the tone control is a dual gang pot, but when I took a photo of the guts, it looks like the volume is the dual gang, rather than the tone pot. Maybe I'm just seeing things wrong?

[sluckey]

The Harmony schematic clearly shows the volume control as being a dual ganged 500K pot. The Gretsch does not show the two volume pots as being linked together.

https://el34world.com/charts/Schematics/files/Harmony/Harmony_430.pdf

https://el34world.com/charts/Schematics/files/Gretsch/Gretsch6162.pdf

Also notice the second volume pot... Harmony has a 100K between the bottom of the pot and ground while the Gretsch simply connects the bottom of that pot directly to ground.

[sluckey]

So, you have a Harmony 430 but you posted a schematic sniplet from the Gretsch schematic. Why even drag the Gretsch into the discussion? Just a red herring?   

[Duncan]

So, you have a Harmony 430 but you posted a schematic sniplet from the Gretsch schematic. Why even drag the Gretsch into the discussion? Just a red herring?   

Well, the guts in my Harmony vary quite a bit from the posted schematic, and is closer to the Gretsch in many ways. I know that Valco amps are often much different than whatever their schematics show (maybe due to undocumented revisions, parts availability, etc), so yeah, figured I'd post the tone circuit schematic from the amp my Harmony is closest to.

[passaloutre]

I’d be curious to see the response of this circuit on a graph. It resembles the Big Muff tone control but with two-pole filters.

[tubeswell]

I’d be curious to see the response of this circuit on a graph. It resembles the Big Muff tone control but with two-pole filters.

Yes. Sort of. With the tone control rotated fully one way it’s two cascaded R/C (2nd order) filters, and fully the other way it’s two cascaded C/R (2nd order) filters. Each following filter stage has an impedance  several orders of magnitude higher than the previous stage to reduce the loading effect. And at center-rotation, the two sets of filters ‘cancel each other out’ wanting to form a bit of a humpy band-pass so to speak.

[bmccowan]

So, you have a Harmony 430 but you posted a schematic sniplet from the Gretsch schematic. Why even drag the Gretsch into the discussion? Just a red herring?   

Well, the guts in my Harmony vary quite a bit from the posted schematic, and is closer to the Gretsch in many ways. I know that Valco amps are often much different than whatever their schematics show (maybe due to undocumented revisions, parts availability, etc), so yeah, figured I'd post the tone circuit schematic from the amp my Harmony is closest to.

I sympathize - I have a National Newport 50 that is very close the 6162 schematic. In researching that circuit, I found that the actual circuits vary quite a bit from each other and from "official" schematics. More so than even Gibson! And based on web searches, this forum and others appear to deal with a lot of hum and buzz problems with H430 amps.
I have scratch built a couple of amps using the 6973 power tubes. One was a 1624 Lite that I built into a AO-39 chassis (Sluckey inspiration there) which I later changed to a 5879 preamp similar to the Gibson GA-40. The other I used the front end of the top boost circuit that Mojo uses in their NC3015 married to the Valco 6973 power amp - it was a shotgun wedding. That amp sounds great.
I mention all this because if you do want to build that circuit - I think you should call it the Red Herring. But if you are looking for the 6973 sound, there are other options that might avoid some of the gremlins that seem to haunt the H430.

[Duncan]

So, you have a Harmony 430 but you posted a schematic sniplet from the Gretsch schematic. Why even drag the Gretsch into the discussion? Just a red herring?   

Well, the guts in my Harmony vary quite a bit from the posted schematic, and is closer to the Gretsch in many ways. I know that Valco amps are often much different than whatever their schematics show (maybe due to undocumented revisions, parts availability, etc), so yeah, figured I'd post the tone circuit schematic from the amp my Harmony is closest to.

I sympathize - I have a National Newport 50 that is very close the 6162 schematic. In researching that circuit, I found that the actual circuits vary quite a bit from each other and from "official" schematics. More so than even Gibson! And based on web searches, this forum and others appear to deal with a lot of hum and buzz problems with H430 amps.
I have scratch built a couple of amps using the 6973 power tubes. One was a 1624 Lite that I built into a AO-39 chassis (Sluckey inspiration there) which I later changed to a 5879 preamp similar to the Gibson GA-40. The other I used the front end of the top boost circuit that Mojo uses in their NC3015 married to the Valco 6973 power amp - it was a shotgun wedding. That amp sounds great.
I mention all this because if you do want to build that circuit - I think you should call it the Red Herring. But if you are looking for the 6973 sound, there are other options that might avoid some of the gremlins that seem to haunt the H430.

Red Herring is a GREAT name.

One thing I plan to absolutely do is, whether through transformer selection or dropping methods, reduce the plate and grid voltage to the power tubes so I can safely run 6CZ6’s in there.

My 430 has a pretty high noise floor (audible hissing at idle) and I would put money on it being a dogs breakfast of grounding and probably some carbon comp resistors doing their thing. Plus there’s absolutely zero lead dress considerations beyond the heater wiring which I redid.

Once I’ve traced what circuit actually lives inside my 430, I plan on making it a PDF and uploading it to as many places as I can, to potentially help other 430 owners with oddball one-of’s.

[bmccowan]

All mine run on 6CZ5 tubes. What I have found in studying those tubes closely, is that the RCA 6CZ5 is absolutely identical to the RCA 6973, except for the pinout. The Sylvania 6CZ5 is not and appears less stout in construction. So I concluded that the RCA tube could take the voltage. There is a thread here about that from 2 or so years ago.
I have several Valco amps, as I bought them years ago before eBay buyers realized that Supros were Valcos. Some are noisy, some are not. I have found most of the noise to be around the PI tube.  As you said a combination of lead dress and ground issues. But I have also found cleaning the tube sockets helped and also trying different tubes in that position including the whole alphabet of 12_7 tubes. Replacing the input jacks helps too, even though its a pain with that shield Valco used.
Interestingly, (well to me anyway) I have rebuilt two Supro Thunderbolts and two Harmony 420s. Except for the cabinet, all 4 were identical. And the Harmony 420 schematic is accurate, and the various Thunderbolt schematics that are out there were not. All 4 were quiet well built amps, but with a large chassis and no trem or reverb, there was lots of room for the dog to eat it's breakfast.

[shooter]

Red Herring is a GREAT

barbequed with Uncle Rays Honey Chipotle sauce 

[PRR]

.... the tone circuit shown like this:

Why be normal??

https://www.nutsvolts.com/magazine/article/copper-cobweb-circuit-construction

[sluckey]

Why be normal??

Wish I had seen this back in the '70s!   

[bmccowan]

OMG - that's getting bookmarked for sure.

[tubeswell]

I want one