Question on the Dynaco ST-35 v.s. the SCA35

[EdStiles]

Does anyone know if the Dynaco ST-35 PC-13 boards are physically the same size as the SCA-35 PC-10 driver boards? I am thinking about rebuilding an SCA-35 and would prefer the 7247 triode input of the ST-35 to the 7199 used in the SCA-35. Both use the same EL-84s, power transformer and outputs.

Would there be any other "show stopper" to making this change? I would replace the power transformer with a Triode PA-774 and replace the PS caps with one of the new cap boards during rebuild.

Thanks,

Ed

[erichayes]

You could use the ST-35 driver boards, Ed, if you're willing to sacrifice the phono preamp stage and all the tone control stuff. The reason Dynaco went with the 7199 driver tube was to get the gain needed to overcome the insertion loss of the tone control circuitry. The phono preamp wasn't all that great to begin with, so I wouldn't consider that much of a loss, but the overall conversion wouldn't be as simple as dropping in the 13s and sitting back and enjoying the results.

I'm sure others will have comments, as this is not the first time the subject has been broached.

[K-MAN]

I've replaced the boards in my SCA35 with ones I got from this guy http://www.dslextreme.com/users/triman/triman.html
I think he's the only guy that makes the driver boards with the 7199, but he also makes the ones from the ST-35. I can't really see the point in switching to the other type of drivers, the 7199 is a real good tube if you get the right one and not the copy made by Sovtek.

[kurtbauer]

No showstoppers to do what you've suggested. I installed ST-35 boards from K-MAN's source and it fits perfectly. I also used Sheldon Stokes' cap board with a mod to adjust bias for each channel. FWIW, I did not rebuild the phono board. Instead I installed a Scott Nixon DAC.

Up to this point, I have not been thrilled with the sound and the gain was anemic (I'm using NOS GE 7247s and a matched quad of JJ power tubes though I only needed matched pairs). Aside from that I had low-level niggling hum. Today I three changes that have made a significant improvement.

First, I had decided to add the hum pots back into the mix. I've put around 14V on a the wiper of a 5k pot for each heater. The result is no audible hum through 90dB efficiency speakers. I'm pretty happy as it's been bugging me for 6 months.

Next, it had been suggested to me that I should get rid of the RC network on the grid and just use a 100 Ohm carbon comp resistor. I didn't have a CC resistor (I'll do that later) but I gave it a try and the gain is improved and the haze is mitigated and I now have a good bass response. No oscillations yet. :-)

Lastly, I changed out the coupling caps. I was using Fast Solens and changed them to Orange Drops on the recommendation of a couple of people. I put in .1 microF/600V and they are kinda big. If they melt, I'll try some 400Vs.

Aside from being hot enough to cook stir-fry on it, I'm really happy with the project now.

KB

[TerrySmith]

Try different tubes instead of the 7247, ie: 12AT7, 5751, 12AX7, etc. Also you can tweak the feedback to alter the gain.

[DerekVa]

Try different tubes instead of the 7247, ie: 12AT7, 5751, 12AX7, etc. Also you can tweak the feedback to alter the gain.

Not sure if this will work - the 7247 is a combo 12AX7 / 12AU7, so it has dissimilar characteristics between halves. On the other hand, if it does work that'd be great as I have some Telefunken 12AX7 tubes sitting in a drawer...

12AX7:
Filament Voltage 6.3-12 V
Filament Current 300-150 mA
Plate Voltage (max) 330 V
Plate Current (max) 6 mA
Plate Dissipation (max) 1.1 W

12AU7:
Filament Voltage 6.3-12 V
Filament Current 300-150 mA
Plate Voltage (max) 330 V
Plate Current (max) 22 mA
Plate Dissipation (max) 3 W

Unfortunately, I can't find mu and resistance figures for either, but I'd be surprised if they matched. Any tube gurus out there know if you can sub different preamp/driver tubes?

-D

[K-MAN]

If your using the ST35 board I think your gain will be less than the old circuit. I've read that tweaking the negative feedback can help when you've modified the circuit like this, less feedback= more gain.

[EWBrown]

I converted an SCA-35 to a pseudo-ST35, using some aftermarket replacement ST35 (7247) boards, which retained the original ST35 circuit design and layout..

I also removed the preamp board, input selector switch and all the PECs, and installed a Hong Kong" stepped attenuator. I also added insulated RCA input connectors, and just left the old RCAs in place to fill the otherwise empty hole in the back of the enclosure.

In place of the preamp board, I made up a cap board, and added a C354 in place of teh original 50 ohm power resistor in the PSU. I staryed with the original single 95 ohm, 5W shared cathode resistor for this one, which mandates using well-matched quads.

Sounds great! Without the messy and long signal path of the original SCA-35, the sound improved very noticeably.

/ed B

[4-CHANNEL]

I know from ebay, a guy sells st-35 pc-13 bare boards, for details, please check from ebay.