VHT Special 6 Ultra - does someone has a Layout to share ?

[Kagliostro]

I've seen Rob Robinette mod for this amp

https://robrobinette.com/Special6UltraMod.htm

I haven't a VHT Special 6 Ultra, but if I've a Layout ......

Thanks

K

[JD0x0]

There's two schematics in that link you shared.

[Kagliostro]

Ciao JD0x0

Yes, and also a big resolution photo of the board with labeled components



but it mean to spend some hours to copy the whole thing and draw an usable layout

that's why I hoped someone would have one ready

K

[jml77]

I don't have a layout, so can't help you there. I have a sp6Ultra, and I don''t find the chassis the easiest to work on, and often thought about putting it into a different box.

If you do go on to build one, you might want to modify the heater supplies. They are grounded on either side of the rectifier, which I've been reliably informed is not good practice. My amp had a lot of buzz, so I set about tackling it, first by going over the grounding, which isn't too bad really, but that led me to change the heater supplies and go for an all AC supply, with a humdinger pot. It's not a good idea to use a humdinger pot with the stock supply, if the pot is fully rotated by accident, it (the pot and maybe more) will go up in smoke. I've found the all AC supply with humdinger much less noisy than it was stock. Even with the higher gain setting it's better on AC than the bodged DC. If doing it again I would consider using a separate small transformer and build a stabilised regulated DC heater supply for the first preamp tube.

I found the buffered fx loop rather useless, far too easy to overdrive the loop, especially if the ultra stage is used. So, I don't use the loop.

[Kagliostro]

Ciao Jml77

Thanks for your contribute

...the heater supplies. They are grounded on either side of the rectifier, which I've been reliably informed is not good practice.

I don't see where ??

May be you refer to the fact that the FWB is grounded, if so, you must remember that the PT Heater winding hasn't a CT

(so no CT ground connection)

--

The fake ground reference, the pair of 100R resistor connected to ground, can be swapped for a 250R pot (with humdinger purpose)

to add an Humdinger without removing those resistor I think will not give the best result

Franco

[jml77]

Maybe it works ok as designed, though I have always thought there was something odd about it. Reading Merlin Blencowe's website and book he states several times that there must be no ground on the Ac side if the DC side has been grounded, or vice versa. Maybe the two 100 ohm resistors and 'fake centre tap save it, I'm sure they do, as it does rectify and appear to work. If in doubt run it in a simulator or scope it out, there's definitely something odd going on .

I wouldn't recommend replacing the two 100 ohm resistors centre tapped to ground with a pot in the stock doubly grounded circuit, if the pot is fully rotated by accident then it will short out the transformer, and burn out the pot. There is a fuse, which might minimise the damage.


Edit: Just to add, I remember reading somewhere that the humdinger was really only effective in reducing hum at the first stage, where it is most effective at reducing heater noise getting into the circuit. So in that case why bother having the two 100 ohm resistors and grounded centre tap for the 6v6, if the first stage is fed with DC heater supply.

[Kagliostro]

If the problem is that the humdinger pot can be rotated to ground one of the winding extremities, as to avoid that, you can insert the pot (150R instead of 220/250R) at the junction of the (100R) resistor, this way you can balance the windings avoiding to short one extremity to ground

--

Having a pair of 100R resistor with the junction connected to ground in a heater winding without CT is something as to have a CT connected to ground using a resistor, this way the resistor acts as a load and there is no a short via the FWB diodes with one branch of the winding (I hope I was able to explain in a comprensible way)

Franco

[jml77]

Yes, I understand what you say. I think that the stock circuit is safe as it is, but I do think it is not recommended. As you say the 100 ohm resistors save it.
I think it is just easier to leave them out, there might not be a need for them at all. I've found removing the bridge and all DC associated components, and using an AC supply with humdinger (hum-balance) simpler. The Buzz has gone. And there is something quite satisfying in turning up the volumes, tweaking the balance pot and trimming out the hum. If using both inputs then there's a certain compromise to be found, the ultra input and clean input do not have the exact same balance point and a change of tubes requires another balancing performance. I tend to balance up the ultra input for minimum hum. The clean input is less of a problem, mostly.

[Kagliostro]

Ciao Jml77

I belive you when you say that you experimented with satisfaction the use on an AC Heater PS

What seems to me a bit odd is the fact that you have different performances on the use of the Humdinger when you use the Normal Channel vs the Ultra Channel

As far as I can know the benefit on the use of a pot (as Humdinger) is due to the fact that often the CT of the Heater Winding isn't perfectly "centered", so, moving the wiper you adapt the resistence value to your winding (when an Humdinger is in use the winding CT must be disconnected)

However what really means is to achieve the desired effect that you reached

Franco

[jml77]

Franco,
It is puzzling, I didn't expect to have to do that. Maybe it's to do with the extra gain when using the ultra side of the valve. I did wonder whether it might be due to the way the heater noise interacts with the spiral filaments and the cathodes in V1. And somehow there's an imbalance between the 2 halves. If I change valves I need to re-balance for minimum hum.
J

[Kagliostro]

Stupid me

Using the ultra channel the amplification is greater, therefore the noise inevitably tends to increase

Similar thing if you change the tube, different tube, different gain (within tube spec of course)

I think that you must set the Humdinger for the Ultra channel, the normal channel will be less noised

Franco

[jml77]

Thanks Franco, It has been good to have this discussion, I've learnt something (I hope). Although I worked as an electronic tech., I know there are some fundamental gaps in my knowledge. I am cautious, so tend to drop down to the easiest solutions which I am more confident about. But I still make basic mistakes from time to time.

Good luck with the layout for the SP6ultra, you should find it easier to work on with a different chassis and layout. I thought the VHT box had too many flying wires, a bit of a rats nest in some ways. Rats nest wiring can be good, but equally can be bad, and it's so difficult to work around without disturbing something and making matters worse. To make things simpler I removed a lot of the amp functions that I didn't or rarely used, ie the triode/pentode switch, the depth control, the watts control, single tone control, moved the loop components to a small piece of tag board, and moved the tone stack components up to added pots for BMT controls. This all helped to reduce the ammount of flying wires. It's still a bit of a mess though.
I'm working on a total rewire and design, 6v6 push pull, redesigned preamp. It's on paper, not sure when I'll commit to making it.

I'll look forward to seeing your finished project.

[Kagliostro]

At this point your job will be easier if you use the Special 6 as start instead of the Special 6 Ultra

I'm working on a total rewire and design

In this case, may be you will find useful to find on the web a good layout for a Fender 5F2 and start to draw your Ultra version from that

Franco

[jml77]

I'm too far down the line to do a 5F2, maybe another time.

I need the space of the ultra chassis, the new amp will be a fairly standard design, trying to keep most preamp stages centre biased, I'm not too keen on the sound of cold clippers and asymmetric clipping, 3 stage preamp, 2 tones stacks with switchable lift, a few switchable cathode bypass options, and a cathodyne PI. I can't make up my mind whether to leave the 6v6 PP stage bypassed or not or whether to have shared or separate cathode resistors, I was thinking of making it switchable. It will be a bit of a mongrel, with ideas pinched from lots of different amps. I have some LND150 mosfets which I might use for fx loop between the last tonestack and the PI, I have one designed and it's simulating well, but I'm in no hurry to include it.

I work slowly, poor health gets in the way. I like it slow...

[Kagliostro]

I work slowly, poor health gets in the way. I like it slow...

I too

Franco