Dynaco ST70 amp with VTA ST-70 CCS driver board

The VTA ST-70 driver board is a replacement for the venerable Dynaco 70 (tube amp) driver board. Read about it from the designer: http://www.tubes4hifi.com/ST70.htm
This page documents this upgrade as the driver board was installed.

The design is solid and well regarded, and as a bonus, it cleans up the under chassis wiring considerably. For example, the the negative voltage bias supply and a trim pot to set each output tubes idle current is all onboard now. (Separate trimmers negates the need for closely matched for plate current output tubes.)

PCB Build tips:

The PCB assembly notes are simple and clear, the PCB is well made and clearly labeled so it goes together quickly.
– For a pro look, arrange the resistors to read in the same direction, and make sure the capacitor values are visible.
– Space the power resistors off of the board about 0.1″. This avoids burning the board and allows max air cooling for the resistors.
– I chose to put the electrolytics and coupler capacitors on the bottom side of the board to get them away from the the heat of the tubes. (See pics below)

– I’ve long been a fan of Solens caps for couplers in Dyna tube amps. Here, I used 2 for the 0.1uF driver to phase splitter and I used 4 1.0uF for the ‘splitter to output’ couplers. The larger values should flatten the bass response a bit from the stock values.
ED NOTE: How much better? Hmmm, need to build second board with default values and see!)


Here’s what my finished chassis looks like from the top.
Observe the old bias pots are unused and removed with this upgrade.
I put both banana and pin tip jacks to fill the holes, wired to the star ground point. This is useful for setting the idle current, bench testing, debugging ground loops, and so forth.
Also added are 4 more pin jacks (red) for setting the idle current per tube.

The underside now looks like this.

You can clearly see the pin jacks, cleaned up wiring, and a few other safety and reliability mods here too. Refer to my general Dynaco 70 page for more details on the mods: Vacuum Tube Audio – Dynaco


I ordered some NOS 6189’s from AES. While they were in transit, I used some old 12AU7’s I had laying around. (They measured fine on my Hickok 600A tester.)
The rectifier tube is one of my precious original Mullards, and the EL-34 outputs are new, current production JJ’s. On first power up, everything looks good, the amp biased up to a 40mA idle, and a sine wave looked clean.

After about 1 hour of run-in, here’s what THD vs frequency looks like at 1 watt.

THD vs power:

Some scope photos…

Square waves at 2 watts looks like:

Dyna/VTA-70 at 2 watts, 440 Hz

Dyna/VTA-70 at 2 watts, 10kHz

These are good results overall. The difficult square wave test shows a tiny bit of overshoot at 440 Hz, and the 10 kHz pattern is quite good too.

The THD is a tad higher overall then I would like, especially in the right channel at low frequencies. I’ll recheck when the 6189 tubes arrive, but this is probably the output tubes being under biased a bit.

Yay! The 6189s arrived (thanks AES!), and I also upped the idle current to 45 mA per tube.
This combination yields this sweep of frequency vs THD response at 1 watt
The frequency response scale is +/-1 dB (left side numbers)
The THD scale is 0 to 1% (right side numbers).

These measurements are now quite good.
The THD is under 0.05% over the critical midrange, and under 0.2% overall except below 30Hz. The rising THD at extremely low frequencies suggest there’s still a bit of strain in the low bass, I would recommend a subwoofer and an active crossover at about 80Hz.
The amp draw about 160 watts while idling, not a bad sounding heater! 🙂

2021.11.26 – Next up:
– Comparing a SS rectifier to a real (old stock) Mullard 5AR4 rectifier
– Comparing new JJ-34’s to new JJ KT-77’s to old stock GE 6CA7 “fat boys”

The B+ rectifier:
Replacing a tube rectifier with a SS rectifier isn’t a simple direct swap.
Some things to consider are:
– Silicon rectifiers have considerably less forward voltage drop than tubes, meaning your B+ will be too high. This must be dealt with, else a dangerous overvoltage will occur.
– 5AR4’s are a slow turn on rectifier tube, which is theoretically less stressful on the high voltage system and tubes. Opinions are mixed as to just how important this is, but a softer turn on is clearly less stressful by some factor.
– The rectifier takes a hefty 1.9 amps at 5 VAC (9.5) watts just for the heater. Removing that load from the power transformer theoretically should make it run a bit cooler.

For this experiment:
– I already installed a thermistor on the transformers primary which should address enough of the potential turn on surge issue, as well as dropping about 2.8 VAC on the primary.
– I already installed the 1000 PIV plate series diodes to address the PIV issues with the current production tubes.
I built my SS rectifier jig on a tube plug with 1000 PIV diodes and a 50 ohm, 10 watt series resistor, thereby allowing a quick swap with a tube rectifier. As usual, I’ll first measure and later listen.

A quick check showed no increase in THD or S/N, and the power output vs THD looks to be the same too. Good enough for now, more measurement and listening tests later.

Output tube comparison measurements

The first wrinkle was the KT-77’s wouldn’t bias up high enough with the stock VTA bias circuit to get up to my currently preferred 45 mA idle current. I’ll reduce the ground resistor to get enough range from the pot. Also, the bias adjustment already feels a little too twitchy with the stock bias trimmers, so I’ll replace them with multiturn units while I’m in there.

Done with bias circuit mods.
– Tacked a 33K across the existing 27K bias-pot-to-ground resistor, which makes R37 & R38 about 15K. This increases the the bias voltage range from the stock range of -60 / -34 to -60 / -24 VDC, which now allows KT-77’s to be biased up warm enough to use.
The multiturn pots I used are 50K cermet 12 turn Bornes units, rated at 250mA. Note they fit perfectly in the current boards, but are “backwards” from the VTA stock units. IOW, a CW rotation increases the negative voltage resulting in less idle current (cooler plates).

KT-77 quick tests

Frequency vs THD is about .1% higher THD on average than the EL-34’S, even after I biased them hotter at 50 mA. The THD is over 0.2% over the entire spectrum, and peaks up to about 0.35% at 7 KHz. Doesn’t seem like the best match so far.

Power output vs THD – The KT-77’s have an odd kink in their THD vs power response.

And here’s the spectrum analysis. Fairly rich in even harmonics, but I’ll need to re-run the EL-34’s for a better comparison.

Tests with 6CA7 vintage GE “fatboys”

Here’s the charts with the old stock 6CA7’s

Wow, these 50+ year old output tubes still measure really well!

Last Updated on 2021-12-06 by Daev Roehr