Dynaco Mark III?
- Thread starter Blackdog61
- Start date
I did the Yellow Sheet mod on my ST70 and have been on the same Sovtek rectifier for several years. I’ll do that on these.
Been considering VTA for these because I have plenty of AX7 and AU7 tubes, the boards are priced reasonably, and there is a lot of support here. Might add a triode/UL switch. First question is whether I paint the chassis or buy new SS.
Been considering VTA for these because I have plenty of AX7 and AU7 tubes, the boards are priced reasonably, and there is a lot of support here. Might add a triode/UL switch. First question is whether I paint the chassis or buy new SS.
Yes to all of that. Different voltage drop is another concern, if its significantly different the load isn't shared equally.
Before you go the VTA route...
This was posted on the Dynaco Forum by Roy Mottram (former owner of VTA) last week:
"essentially - Dan is not actively selling anything except a couple of PCB on ebay
if you made an order or payment, hopefully you did it with PayPal, your bank, or credit card.
Contact whoever you made payment with and get your $$$ back.
I have had at least six people tell me they NEVER got what they ordered after waiting over a year, no response at all"
(Dan is the current owner of VTA.)
This was posted on the Dynaco Forum by Roy Mottram (former owner of VTA) last week:
"essentially - Dan is not actively selling anything except a couple of PCB on ebay
if you made an order or payment, hopefully you did it with PayPal, your bank, or credit card.
Contact whoever you made payment with and get your $$$ back.
I have had at least six people tell me they NEVER got what they ordered after waiting over a year, no response at all"
(Dan is the current owner of VTA.)
Let the record show that I posted an emoji depicting a brown pile of steaming waste in post 65. It obviously didn’t survive the process.
Lavane
Lunatic Member
Hopefully Roy is selling the mono block version too. The board for the VTA ST-70 and VTA ST-120 are more popular though since there are more of rhe ST-70's.
If Roy is selling the board kits, I may snag one of the 6SN7 boards foe the ST-70.
<<EDIT>> I saw the 12AU7 board only is listed on ebay by Tubes4hifi. It is looking like Bob Latino's and Roy's old business isn't doing well, or has failed under new ownership. There used to be both board kits for sale on ebay. I didn't look further to see if any of the other oprional parts and kits were still listed.
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Thanks for the tip. I just bought two. Looks like I will have a more modern version after all.
@Blackdog61 I assume you mean Mk3 boards. Which did you buy, noval or octal? Either way they are set to run at a pretty low level of feedback. I bumped my Mk2 6SN7 boards up as much as practical but there just isn't enough gain for a higher amount. I think they had about 3dB in the beginning and I was able to get to 5 or 6 before the first stage couldn't drive them to full power. If I do it again, and I likely will since I have pair of Mk3 rat rods, I will probably convert the first stage to use a 6SL7 or, if I'm more energetic, a 6AQ7 which will require cutting some traces and adding a jumper or two. These boards parallel the two sections of the input tube which may or may not be a good thing. It will double the stage gm but will also double the Miller capacitance of that stage.
Couple more ideas. The bias supply isn't filtered very well and has a fair bit of ripple on it some of which will cancel in the PP output stage but not all of it unless your power tubes are a perfect match. With the low feedback you won't get much cancellation there either. The bias pots can also be improved by using some 4 turn Bourns pots which I've used on a couple of amps using the Dave Gillespie IBBA board design. Much less fussy to get your setting dialed.
Stock power supply.
even just adding another 50uF to ground where the yellow spot is will make the bias supply filtering much better with minimal effort.
In theory it could have one from pot wiper to ground too, but that makes the bias adjustment "laggy" as the cap charges and discharges with pot movement.
I would do a 47K from the 1K end of the pot to the center wiper though. That will keep the tubes at full negative if the wiper lifts instead of going to zero volts.
even just adding another 50uF to ground where the yellow spot is will make the bias supply filtering much better with minimal effort.
In theory it could have one from pot wiper to ground too, but that makes the bias adjustment "laggy" as the cap charges and discharges with pot movement.
I would do a 47K from the 1K end of the pot to the center wiper though. That will keep the tubes at full negative if the wiper lifts instead of going to zero volts.
Octal. Should have grabbed all of those NOS RCAs at the estate sale last year…….. I think they were maybe ten bucks each or something.
me, my soldering iron and meter might need to circle back on the details. I’m more of a match the picture and watch for fire guy.
me, my soldering iron and meter might need to circle back on the details. I’m more of a match the picture and watch for fire guy.
All you need to do on the Mk3 octal board, depending on the parts list, is increase R39 between the two bias filter caps. It would help to have a schematic for the octal board. I bought mine from an eastern European seller that makes/markets the auto bias boards. It was a driver board/bias board combo. I think his original R39 was less than 20 ohms. Changing to 470 ohms reduced the ripple on the bias voltage quite a bit.
Here's ripple with the stock resistor value at 0.5V vertical scale.
And after increasing R39 to 470 ohms. Same vertical scale.
Here's ripple with the stock resistor value at 0.5V vertical scale.
And after increasing R39 to 470 ohms. Same vertical scale.
