slow AC-30 clone build.

[martin manning]

2. There is the Merlin Blencowe mod where he puts a diode and resistor from grid to cathode on the cathode follower to prevent arc over. In one circuit there is a 100 ohm resistor between gain stage plate and the followers grid. I looked at his book too and I don't see the significance of that resistor. Any thoughts on the purpose of the 100 ohm resistor?

It's a grid stopper, and I think the value may be in error as 100R seems too low.

edit: From AC coupled cathode follower page: "A grid stopper is added as a matter of course to discourage oscillation. Any value from 1k to 10k is fine, and even larger values won't hurt since the input capacitance is so puny."

[Mark]

It is an odd value for a grid stopper, the smallest I've used is 1K. I don't see what is would be doing either.

I've had yet another crazy idea, as I'd like a LarMar master volume. I could reduced the vib/trem controls to one pull pot or switch. I still may have to use a switch as I seem to remember these circuits use reverse audio taper pots and have large values such as 3M ohms. The speed switch does only go as high as 1 meg which suggests a pot could work though it smallest resistor is 330K. All good for thought.

A hum balance pot is yet another idea. This has worked well on an Ampeg SVT, though I don't know if it would be any better that using the centre tap for the 6.3 filaments?

[Colossal]

It is an odd value for a grid stopper, the smallest I've used is 1K. I don't see what is would be doing either.

I have seen 100R-1k or so used on the grids and cathodes of cathode followers in hi-fi applications. I think the added grid resistor is just there to help stop any oscillation but not affect the waveform, distorting it. Maybe kind of how KF used a very small value (820R) on V1b and/or V1a just to tame the beast a little.

I've had yet another crazy idea, as I'd like a LarMar master volume. I could reduced the vib/trem controls to one pull pot or switch. I still may have to use a switch as I seem to remember these circuits use reverse audio taper pots and have large values such as 3M ohms. The speed switch does only go as high as 1 meg which suggests a pot could work though it smallest resistor is 330K. All good for thought.

I was always a proponent of the LarMar master volume up until somewhat recently. I have been working on a 2xEL84 design for a while. A LarMaster is a useful tool, and while it does reduce volume, I was rather surprised at the noise it can add. Not a horrible, glaring buzz or something, but definitely noticeable with and without (even with good lead dress). Enough to make me really think about it. I built out another two channel 18W that is my favorite yet. It has a paralleled triode channel (into one side of the PI) and the other is a two triode cascade (into the other side of the PI). Both channels have the typical 18W Vol and Tone controls. The grid leak resistors are fixed in that amp, no LarMaster. The nature of that paralleled triode is such that the sound is very linear whether turned down low or turned up. I just set the amp's Vol and Tone, and then use my guitar's volume and tone controls to get where I need to be. I find I don't even need a PPIMV with that amp and it is better and quieter for it. That said, if I was going to build a 100W Plexi, sure, I'd drop a PPIMV in it without hesitation so I think it's about the amp and the intended use.

Over time, I am finding that my preference is for the most stripped down circuit. When I first read Greg Fyer's BMAC30 pages years ago, I was very interested because all Brian May is using is 1/2 triode into a PI, no tone controls (+6dB gain). I am very much enamored with the Songwriter 30 and the parallel triode-into-a-PI and EL84s topology. I daresay I like it even more than the Rocket. So, my point of all this rambling...with your AC30 build, Mark, are you looking for an exploration platform to try AC30 ideas, or go with the stripped down May Minimalist design, but incorporating only those features that are true advancements to stability and reliability, while cutting all the rest of the fat?

A hum balance pot is yet another idea. This has worked well on an Ampeg SVT, though I don't know if it would be any better that using the centre tap for the 6.3 filaments?

In previous endeavors to build complicated amps , I tried a hum balance pot on a number of them, push-pull and single ended. I found that it didn't seem to do much! The heater windings were always balanced anyway and even unbalancing them with the pot didn't seem to make the amp hum. So, I don't use a balance pot at all, but do almost always elevate the heater centertap to DC, especially if there is a CF in the amp.

[Mark]

Thanks for you advice Colossal, any thoughts about caps and resistors?

[pdf64]

I've had a few other wild and wonderful thoughts, these being:

1. Wire the cathode follower circuit up so it can also use a 12DW7, this valve has one 12AX7 triode and a 12AU7 triode. From what I've read the 12AX7 takes a real beating in the cathode follower position...

The CF triode of the DCCF in an AC50 does tend to have its plate dissipation pushed but that's due to the high HT (eg up ~450V). It's just not an issue in an AC30, where it's only being fed <300V.

...There is the Merlin Blencowe mod where he puts a diode and resistor from grid to cathode on the cathode follower to prevent arc over...

I think that idea is of no benefit in an amp with a GZ34 rectifier; the signal tubes will have warmed up and be ready to conduct by the time the HT has ramped up to any significant level, hence there's no potential for arc over to occur.

...As stated before the power transformer has a 303 - 0 - 303vac secondary...

That's crazy high, useless for an AC30 IMO.
I don't see the point in starting this unless you can get a more suitable PT.
The Woden in mine puts out 586Vac with the HT unloaded, 560Vac at idle, 560Vac at max 'clean' 25W, 554Vac at fully overdriven squarewave 38W. The HT idles at 317Vdc.
243Vac mains feeding 245V PT primary, heaters 6.41Vac at the EL84s.
Measurements taken with calibration checked Fluke 189.

I guess that if you've already got that PT, yes you could try bringing the voltage down, either with ~100ohm 10W in series with each HT winding phase, or a suitable zener, but that's adding yet more heat to an already hot amp.

[Mark]

Did I say it was going to be a slow build?

I should get more screws to put it together this week. I'll start wiring and just peg some parts together to get an idea of rail voltage, plate and screen dissipation. I think I have Zener to lower the voltage which might do the trick for the purpose of measuring etc.

IMG_1007.JPG

Nothing to be proud of yet, but it is a start.

[Tony Bones]

You should make a YT vid of the build, but spend a lot of time talking about irrelevant shit so that the vid ends up 6 or 7 hours long.

Just kidding. I hate those vids. Don't do it. But do keep us posted with pics. Doesn't matter how long it takes.

[pompeiisneaks]

You should make a YT vid of the build, but spend a lot of time talking about irrelevant shit so that the vid ends up 6 or 7 hours long.

Just kidding. I hate those vids. Don't do it. But do keep us posted with pics. Doesn't matter how long it takes.

Sounds like you're talking about my build videos ;D

~Phil

[Tony Bones]

Ha! No I enjoy your vids. They move along at an easy pace.

[Mark]

Whilst working out what transformers I wanted in the amp I corresponded with Don Butler, he's a great guy and was very helpful.

MMP-V30WIt has a real good supply platform and is very quiet. B after rectification is around 350v d.c. For a clean Vox, I like an amp with a B in this range. You usually end up with about 33 watts (RMS) @ clipping.Regards; DonToneman, Inc.

Hopefully, in the next few days I'll receive hardware through the post which should see more stuff on the chassis.

[Mark]

Have another parts question. Does anyone know who sells the 8uF/8uF can caps Vox used?

I thought TAD sold them, but apparently they don't.

[sluckey]

Here's some F&Ts. They're in Australia too...

https://www.ebay.com/itm/F-T-8uf-8uf-45 ... SwvBpbTC9o

Oops! Looks like the 8/8s are probably the axial style.

[Mark]

Thanks for looking for the 8uF dual can cap for me. I did see these but they are an axial cap with leads, while I was shooting for one like the original with tags at one end.

https://hfc-fs.s3-eu-west-1.amazonaws.c ... az-800.jpg

[M Fowler]

https://www.parts-express.com/parts-exp ... r--020-603

[Mark]

Slowly it moves on. The earth is attached to the bracket holding the filter cap in place. I didn't like that idea much so I will have a dedicated earth.

IMG_1009.JPG