get*set*go 6AV5GA build

2nd harmonics for the masses

get*set*go 6AV5GA build

Postby EWBrown » Fri Dec 24, 2010 10:25 am

Sooner, or later, it had to happen. A G*S*G with 6AV5GAs in place of 6B4Gs. The rectifier is a Sylvania 6AX5GT and for the VA, a Tung-Sol 6SL7, all of the tubes NOS.


Image

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Those with sharp "eagle eyes" will notice that the plate is still labelled for the original 6B4G / 6C4C triodes, next to the octal sockets.


I took my original G*S*G which was built on Front Panel Express plate, and re-wired the tube sockets for 6AV5GAs. The cathode is connected tio the "test point" for each channel, which is the "top" of the cathode resistors and bypass caps on the PC board.

I connected the plate to the screen grid with a 470 ohm, 1W resistor, at the socket.

I kept the DC filament power for now, but may eventually re-wire it for AC just to lighten the load on the PA774's filament windings (the rectification ptocess consumes 1.4 to 1.8X the delivered current).

Powered it up, made a few measurements, and the new tubes were within a volt or two, and within a milliamp of the 6B4Gs.

Connected it to the system, and it sounded every bit as good as the 6B4Gs, with somewhat better bass response.

Since the tubes have a screen grid and my James 6112HS OPTs have a SG tap, I just had to try ultralinear operation.

It definitely had a lot more output power, and even better bass response, but the high freqs sounded just a bit too harsh, a sign of higher THD.
Might be good for a "Champ" guitar amp, or listening to death-metal, (666) (666) (666) but it was kinda rough on the ears, otherwise

OK, back to triode mode, it went back to the noticeably cleaner sound quality.

My measurements indicated the tubes were drawing 49 to 50 mA, with the 900 ohm cathode resistors. I decided to kick things up just a bit, and lowered it to 800 ohms, by soldering a 7.5K 2W resistor across one of the 2.7Ks which made up the 900 ohm RK on the board. It seemed to make a slight improvement, but I wasn't going to take the resistors out and compare again.

I should have just changed one channel, then I would have had both available for immediate comparison, but then hindsight is always 20-20...

One more test, in the spirit of Tube Lab George...

http://www.tubelab.com/6AV5.htm

I took an empty octal base, and made a plug in 6AX5GT substitute, using 2 UF4007s with a 200 ohm, 5W WW resistor in series with each anode.
This delivered a B+ of 330VDC, and the 6AV5GAs wer e now consuming 60 mA, and with a V P-K of 268V across each one, for a plate dissipation of 16 watts. This also took 1.2A of filament current demand off one of the filament windings.

Tried it out, it sounded a LOT better, much more "defined" bass, and clearer highs. I ran it this way for a couple hours, and the tubes didn't show any "distress" and the PA774 stayed warm to the touch, and not blisteringly, scaldingly hot as in an ST35.

Next step, take out the 200 ohm series resistors, and kick up the B+ to around 360VDC and see & hear what happens.... I finally decided on 50 ohms 5W in series with each anode, as being the best of both worlds.

I've got LOTS of 6AV5GAs (and 12AV5GAs), so I am not worried about cooking any tubes. Not all 6AV5GAs are created equal, some RCAs have rather wimpy plate structures, other brands have larger, heavier plates. I've been using the GEs for now, and will try sme DuMonts (made by Sylvania). I'll dig thru the stash and see if I have any 6FW5s, they are similar to 6AV5GA, but with an 18W plate dissipation rating.

All of this is basically a "test bed" for another upcoming design using 12AV5GAs and SRPP 12SL7s or 12AT7s. A good way to kick off the new year! (lol) :))


/ed B in NC
Last edited by EWBrown on Sat May 10, 2014 8:21 pm, edited 1 time in total.
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Get*Set*Go 6AV5GA with different PT, rectifier?

Postby zman01 » Tue Mar 18, 2014 1:34 pm

Noob making first post at the diytube.com forum... :)

Have been reading up on the GSG amp over the past few days. I had initially planned to start with the 6B4G as per Shannon's original design, but found out that the price of these tubes have gone up considerably (credit probably goes to GSG? *) ). I searched for substitutes and came across the 6AV5GA build by Ed Brown and now thinking in that direction.

Well, I have a PT and a pair of OPTs that I would like to utilize for the project.

The PT is the Edcor XPWR-066, which is 600V (300-0-300) at 200mA center tapped, 6.3V at 4A and 5V at 3A.
https://edcorusa.com/xpwr066

OPTs are Edcor GXSE-5k, 15 watts, 6 ohm secondaries. I can also try and get a James with 3.5k/5k primaries.

On the PT I notice a big change from the PA-774 or PB-028 (240 volt version) that are recommended:

- Lower voltage: 300-0-300 vs 330-0-330
- Different taps: 1 x 6.3V @ 4A, 1 x 5V @ 3A vs 2 x 6.3A @ 2.5A

My limited understanding point me towards following solutions:

- Will need to change the rectifier to a 5V rectifier like the 5AR4
- Since PT voltage rating is lower, need a rectifier with less voltage drop - i.e. the 5AR4 should work as it will result in roughly 40 volts less drop vs the 6AX5?

I am also assuming that:

- 1 x 6.3V @ 4A tap should be enough for 2 x 6AV5GA
- The 6SL7 should work fine in this setup

Is this is a workable configuration?

Regards,
Zia
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Postby EWBrown » Thu Mar 20, 2014 5:30 pm

trying again..... lousy connections tonight.... (evil)
Last edited by EWBrown on Thu Mar 20, 2014 5:47 pm, edited 1 time in total.
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Re: get*set*go 6AV5GA build

Postby EWBrown » Thu Mar 20, 2014 5:40 pm

That should work, the 6AV5GA filaments are 1.2 amps each , the 6SL7 300 ma. Supply them 6.3VAC directly, no need to rectify to DC first. Delete rectifiers D1 through D8, and Resistors R19 and R20, they are not required.

The 6AV5GA cathodes connect to the top end of the (900 ohm) cathode resistors, and the two resistors connected across the (6B4G) filaments are unnecessary.
If the 6.3VAC winding has a center tap, connect it to the power ground at J6 pin 2 or 3.

A 5VAC filament rectifier tube can be used, typically the 5VAC connects to V2, pins 2 and 8, use pin 8 for the rectifier (cathode) B+ output to the filter caps and choke.
The two "hot" ends of the 300-0-300 VAC secondary should connect directly to pins 4 and 6. The easiest way to implement this is to solder the leads directly to the rectifier tube's socket pads on the solder side of the PC board.

An alternative, if you like to use the terminal blocks, is to connect V2 Pin 3 to Pin 4; and Pin 5 to Pin6., this converts from the 6AX5GA, to commonly used 5V rectifier base pinouts.

This also allows trying different rectifier tubes, like 5AR4/GZ34, 5U4GB, 5Y3GT, etc.. If the 5VAC winding has a center tap, do not use it, cut it short, and cap it off with shrink tubing or a small wire nut, in order to prevent it from shorting out to anything.

NOTE: In order to use a 5V rectifier, the etch connected to pin 2 MUST be cut, and have about 1/4 inch / 6 mm gap cut in, in order to prevent arcing over of HV to the 6.3 VAC filament power.

I am going strictly by memory, and I am also experiencing weird connection problems, so I will stop for now, just carefully check your schematic and verify that these directions are correct, before proceeding. If I see any mistakes / errors, I will correct them.

/ed B
Last edited by EWBrown on Tue Dec 30, 2014 7:02 pm, edited 1 time in total.
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Re: get*set*go 6AV5GA build

Postby zman01 » Thu Mar 20, 2014 10:58 pm

Ed,

Many thanks for the detailed reply. Good to know that this doable, and will definitely update as I make progress.

- Zia
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Re: get*set*go 6AV5GA build

Postby EWBrown » Sat May 10, 2014 8:50 pm

6AV5GAs, Part Deux: viewtopic.php?t=4576
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Re: get*set*go 6AV5GA build

Postby zman01 » Mon Jan 12, 2015 12:54 pm

Finally the GSG with 6AV5GA is complete with all the EWBrown tweaks.

Very nice sounding amp indeed. I haven't heard GSG incarnations with 6B4G, 2A3, or 300B tubes, so I cannot make any comparative statements, but I like what I am hearing. As described by Ed and few other folks who have done these builds earlier, the mid-range and highs are transparent, and the bass is very impressive for a SE amplifier.

I went for the stock BOM power transformer - the PB-028 which is the international version of the PA-774. The tubes are GE 6AV5GA - they look like the ones in Ed's pic in the first thread; input tube is a Tung Sol 6SL7 (new issue), and using SS rectifiers with 50 Ohm resistors in series. Output iron is Edcor GXSE-3.5k-6.

Thanks to Ed and others for their help.
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Re: get*set*go 6AV5GA build

Postby jwhitmor » Wed Nov 02, 2016 3:45 pm

I was re-reading this topic, because I too have a 6AV5GT build (everything else is stock BOM). Since others will read these really informative postings as well, I want to ask one question:
If zman01 had really decided to go with a different transformer (the one with the 5V winding for the rectifier tube) would it really have been OK to power both 6AV5 cathode heaters from the same 6.3 volt winding? My opinion is, that the 6B4G tubes won't work that way, because the heaters ARE the cathodes, but it should be OK with the 6AV5GT indirectly heated cathodes, since the heaters do not require a connection to the actual cathode. Probably worth making this clear if that is the case.
Thanks,
J.W.
If it is not making X-rays, your B+ is too low.
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Re: get*set*go 6AV5GA build

Postby EWBrown » Fri Nov 04, 2016 8:54 am

The two 6AV5GA heaters can be powered from the same 6.3VAC winding, no problem. Each consumes 1.2A continuous, so plan accordingly,
I'd dedicate at least 3A for the two 6AV5GAS, just to take it easy on the power transformer. The 6SL7 can also use 6.3VAC, as it does in the
original design, IIRC it consumes 0.3 amps..

HTH

/ed B in NC
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