Try the 26 filament with the filament regulator adjusted for 1.5V at first. Some samples of old tubes will require full voltage to achieve the rated gm. This is more likely if the extra anode voltage was helpful. Plenty of listening before trying lower voltage, but a small reduction can help with microphony .
Thanks! How is it with the M2? I have an M2 as well. So far, really enjoying it with the F4.
Thanks Rod! I was at 1.22 V with about 850 mA. At about 1 amp with 1.5 V. I have two parallel 20 ohm, 10 W Welwyn resistors. They are really cooking. I think I need to get more. Maybe some 30 and 40 ohms to parallel more.
Yup, I'm going to get those based on your recommendation a few posts earlier. I have some 0.412 ohm 50 W Dale resistors, mainly for Pass amps. Didn't have a 10R value on hand, so will have to order. Until then, the Welwyn ones will have to cook. I don't see any browning or signs of smoke yet, but can't leave the thing ON without observation.
I'm a fan of Welwyn WW (W2x) resistors.
Unfortunately they are inductive, but non of them ruined even at extreme circumstances.
I lost several Mills 12W resistors even at 6W dissipation (they stretched).
The Welwyns heats like a stove (once I measured 160C!), but when it cools down, it's like the original.
Unfortunately they are inductive, but non of them ruined even at extreme circumstances.
I lost several Mills 12W resistors even at 6W dissipation (they stretched).
The Welwyns heats like a stove (once I measured 160C!), but when it cools down, it's like the original.
Good to know about the Welwyn resistors.
I experimented with different filament currents and voltages today.
- 1.5 V and close to 1 amp: definitely sounded harsher and less linear.
- Dialed it back to 850 mA: a lot better.
- Upped to 900 mA: even better. The liquidy-ness is back. Very smooth and linear sounding. Filament voltage falls to about 1.22 V again.
This tells me that more experimentation is needed, probably on the test bench with some analysis at different anode voltages and filament currents. Hope to get it to it in the coming days. Although, it will be very hard to take the preamp out of the system. It is so good.
I experimented with different filament currents and voltages today.
- 1.5 V and close to 1 amp: definitely sounded harsher and less linear.
- Dialed it back to 850 mA: a lot better.
- Upped to 900 mA: even better. The liquidy-ness is back. Very smooth and linear sounding. Filament voltage falls to about 1.22 V again.
This tells me that more experimentation is needed, probably on the test bench with some analysis at different anode voltages and filament currents. Hope to get it to it in the coming days. Although, it will be very hard to take the preamp out of the system. It is so good.
Also, are the curves for the 26 at different filament voltages posted somewhere?
Other than sweeping through different points on the graph and measure distortion and other things, is there a more optimum way to find the sweet spot? I would imagine you almost need curves for each tube to select the sweet spot.
Other than sweeping through different points on the graph and measure distortion and other things, is there a more optimum way to find the sweet spot? I would imagine you almost need curves for each tube to select the sweet spot.
@amandarae: Nice! So, are any of the tube amps a better match with this linestage?
@andy: me too. Coming out of the DAC, it goes to the 26, followed by the source follower, followed by the F4. The F4 has zero voltage gain, only current gain. I am counting the DAC as a gain stage. It is possible to strip the input unity gain buffer in the F4 and I might try that one day.
@andy: me too. Coming out of the DAC, it goes to the 26, followed by the source follower, followed by the F4. The F4 has zero voltage gain, only current gain. I am counting the DAC as a gain stage. It is possible to strip the input unity gain buffer in the F4 and I might try that one day.
Last edited:
Interesting how everybody has been telling me for decades how crap my amplifiers are using swinging inductor transformers, yet here we are with people raving about the same topology.
Funny old world, what!
Well, I've heard your amps as you know and I thought they sounded really good. I may contact you if I go ahead with a build - I have all the parts.
Now in working condition (the cannibalism devours its victims):
EL84 SE, EL34 PP, 211 SE, 300B SE, 801a PSE ... E55L spud SE is under preparation...
In everyday use for years: 801 preamp, 300B SE amp.
Thanks!
Yeah, the MoFo seems well-suited for the job. The F4 is push-pull, but if you look at its distortion spectra, it passes the Hiraga test of monotonic decrease is distortion products.
Anyone know off the top of their heads (Ale?) what sort of input voltage headroom the Hybrid Mu Follower boards (a.k.a. Gyrators) need to:
A: function properly?
B: function well?
It seems through (accidental) experimentation that they stop stabilizing voltage at about 20V over the output voltage.
Though, Mr. Moglia does not address this explicitly in the gyrator pages, I notice on the Bartola website that as little as ~35v over the output voltage is mentioned in the guidelines for building the 01a preamp.
How much is too much?
I assume that higher currents, coupled with higher input voltages would require heatsinking on the MosFet M3.
Thanks in advance for your advice!
Best regards,
John
A: function properly?
B: function well?
It seems through (accidental) experimentation that they stop stabilizing voltage at about 20V over the output voltage.
Though, Mr. Moglia does not address this explicitly in the gyrator pages, I notice on the Bartola website that as little as ~35v over the output voltage is mentioned in the guidelines for building the 01a preamp.
How much is too much?
I assume that higher currents, coupled with higher input voltages would require heatsinking on the MosFet M3.
Thanks in advance for your advice!
Best regards,
John