DR103 preamp - what does the CF do?

[bea]

Here is one: Mywatt 200 - Weber-Amps Kompromisslose Einkanäler

Apparently even the 400 W-Version of this maker is also equipped entirely with 12AX7s.
I noticed this by chance and was really surprised to see this.

To my understanding, the larger gain of the stage should result in a larger open loop gain of the driver/pi stage - if the load of the power stage is not too heavy. This should result in a cleaner tone, but also a sharper switch to overdrive and a fuzzier tone.

My goal is actually understanding the tonal differences between the Mywatts and the Hiwatts.

And indeed, i am going to swap in an ECC81 as soon as my current health issues permit (i may not carry heavy loads fir the next time).

[mhuss]

I'm not sure who made the MYWATT line (Weber?), but I'm pretty sure they were located im Deutschland, and they didn't have anything to do (other than copying the basic design) with the various HIWATT companies. So, whatever MYWATT used, they did on their own.

That said, as long as they used the power tube pre-drivers, as in the DR405, the PI drive requirements would not be as stringent as even a DR201, and a 12AX7/ECC83 would probably work fine. I really don't know what the DR405 amps came with from the factory for a PI tube.

[bea]

I'm not sure who made the MYWATT line (Weber?), but I'm pretty sure they were located im Deutschland, and they didn't have anything to do (other than copying the basic design) with the various HIWATT companies. So, whatever MYWATT used, they did on their own.

Yes, that's right - almost: Mywatts are still being made. The company is a one man shop lead by a nice guy in his best ages (third half of the life) - that's why the prices are comparatively low. And Max simply took the 4-Input-Early-70s preamp layout from Your site and combines it with the three power stages (and with an improved bias circuit that can be adjusted from the outside).

The amps sound really good. People who know both the Mywatt and the Hiwatt say the Mywatt will sound harschly when overdriven - which is true - and i would like to figure out why.

[mhuss]

Once you can physically deal with the amp (heal quickly! ), it would be easy enough to try the ECC81 and see if it makes a difference. We can talk theory, but the proof is in the pudding. Is the power supply the same as a DR201?

[bea]

Is the power supply the same as a DR201?

I do not have the circuitry.

It seems to be modeled closely to the original design but there seem some differences. The most obvious one is a toroidal transformer (which probably contributes a lot to the weight of "only" 22 kg). I do not know if the transformer is oversized a bit with respect to the partridge, but at least the voltages seem correct.

(heal quickly! )

Thank You. I'll probably need a surgery to cure my problem. Looks like a routine case, but we are having waiting times in our hospitals.

For the time i am happy about my compact vintage amps (2 Dynacord tube PAs from the 60s slightly modified as bass amps) which i can carry along if i'm careful.

[cyclone]

Yes, that's right - almost: Mywatts are still being made. The company is a one man shop lead by a nice guy in his best ages (third half of the life) - that's why the prices are comparatively low. And Max simply took the 4-Input-Early-70s preamp layout from Your site and combines it with the three power stages (and with an improved bias circuit that can be adjusted from the outside).

The amps sound really good. People who know both the Mywatt and the Hiwatt say the Mywatt will sound harschly when overdriven - which is true - and i would like to figure out why.

My orig Hylight DR103 sounded harsh in overdrive until I used a bucking transformer to drop the ac supply by 12vac. When I did this, there was no longer a need for adjustable bias (with the 90's teslas) all dc voltages suddenly matched the old schematics, the tone drastically improved. It was NOT subtle. Thats one of the problems with copying old amp designs blindly.

[Pedro Vecino]

12V AC subtracted seem to me few volts to compensate settings up to 50 milliamperes through the plates of conventional EL34 in Hiwatt DR103. If I remember correctly, the average current in the original Tesla EL34 was located lower than in other models (possibly cooperate positively in your case).
I've always had to build new bias circuits in these amplifiers to place them at about 33/34mA with 485 volts. No more.

[bea]

12V AC subtracted seem to me few volts to compensate settings up to 50 milliamperes...

... it will not compensate the bias issue. It will increase it: reducing the rail voltage by some amount will also reduce the absolute value of the bias voltage which in turn will lead to a larger quiescent current.

[Dave H]

... it will not compensate the bias issue. It will increase it: reducing the rail voltage by some amount will also reduce the absolute value of the bias voltage which in turn will lead to a larger quiescent current.

Reducing the rail voltage will also reduce the screen voltage. Won't that reduce the bias current?

[Pedro Vecino]

I did not understand well. I thought he used a 12VAC transformer connected out of phase with the high voltage secondary to decrease it. But it refers to a transformer connected out of phase to the primary to reduce internally all voltages.

My orig Hylight DR103 sounded harsh in overdrive until I used a bucking transformer to drop the ac supply by 12vac. When I did this, there was no longer a need for adjustable bias (with the 90's teslas) all dc voltages suddenly matched the old schematics, the tone drastically improved. It was NOT subtle.

[bea]

Reducing the rail voltage will also reduce the screen voltage. Won't that reduce the bias voltage?

Which Effect is the larger one?

[Dave H]

Which Effect is the larger one?

I checked the EL34 data sheet. It's hard to see but I think you are correct. The control grid has the larger effect. I'll go back to my cathode biased amps now.

Edit 1:
LTSpice tells me that reducing the mains voltage by 5% increases the EL34 plate dissipation by about 3W (23W to 26W)

Edit 2:
Wait, I made a mistake. I changed the B+ by 5% but the bias voltage by 10%. If I change both by 5% I now have the dissipation decreasing by 3W when the mains increases 5%
I'll have to recheck.

[bea]

Wait, I made a mistake. I changed the B+ by 5% but the bias voltage by 10%. If I change both by 5% I now have the dissipation decreasing by 3W when the mains increases 5%
I'll have to recheck.

That's the effect i assumed: the bias voltage will also get more negative. This will reduce the current through the anode. Apparently that effect is larger - and that makes it fairly safe to run an old amp designed for a rail voltage of 220 V at 230 V. Good to know - some of them have a fixed non adjustable bias (like the Echolette M40).

[bea]

Please let me pick this up again, including my idea that the open loop gain of the power stage should be increased by usin an ECC83 as a PI instead of an ECC81.

Theoretically, the 12AX7/ECC83 has more gain, but in practice the gain is pretty much determined by the circuit values. IME the 12AT7/ECC81 works better in this position, but for a 50 or 100 it doesn't make much difference until clipping, at which time all bets are off and it's up to what sounds best to you. I don't think I've ever seen a 200 with a 12AX7 in the PI.

I just did some rough math: in the 200, each system of the PI sees a load of about 40 kOhms (90 k Anodes || 70 k at the KT88s). At this load, the ECC81 should already deliver a (slightly) larger gain than the ECC83 due to its much smaller output impedance. So my assumption cannot be correct.

The 400 actually has 12AT7 pre-drivers for each bank of tubes after the PI.

But the 400 operates the power tubes in grid current mode, won't it?

[mhuss]

But the 400 operates the power tubes in grid current mode, won't it?

The AT7/ECC81s are DC coupled to the KT88 grids, and so control idle bias as well. However, they are fundamentally just providing low impedance/higher current drive, like any cathode follower.
(For those unfamiliar: http://hiwatt.org/Schematics/DR_400wOutput.pdf)