Hi Andy,
How you getting on with this amp? I was thinking of a quick lockdown project from the parts bin. I have a couple of pairs of 2a3 (and 45) and a pair of 126b. Was thinking of using the 126b as plate choke, maybe with primary and secondary in series (as was mentioned), passing 15ma. I dont have Coleman boards, was considering just bridge CRC from 10v filament TX for 2.5vdc. (I dont want to buy anything for it.) What do you think? Worth a punt?
The Hammond 126B is more suited to a 2a3 or 6C4C. The 126C is for DHTs at 15mA or less.
If you do some simple calculations you'll see that a resistor loaded valve with any real current through it would need a lot of volts across the resistor. e.g. even with 8mA a 27K resistor would need 216v across it. If you don't have the HT available and want higher currents, an interstage or plate choke is a good option, or alternatively an active load or "gyrator".
Thanks for your reply Andyjevans.
But why 27k ? That's enormous.
I tried 3k with a 6p21s:
B+ is 250v (regulated)
Current value is 38ma.vg=-3v
Filaments current is 1,5A
Filaments voltage is 3,15v.
Output impédance is theorycally 1,6k unless I'm mistaken.
It isn't certainly the best DHT preamp but it works,so ?
But why 27k ? That's enormous.
I tried 3k with a 6p21s:
B+ is 250v (regulated)
Current value is 38ma.vg=-3v
Filaments current is 1,5A
Filaments voltage is 3,15v.
Output impédance is theorycally 1,6k unless I'm mistaken.
It isn't certainly the best DHT preamp but it works,so ?
Last edited:
I mentioned 27K since it would suit a DHT like the 26. A 6P21S has a Ri of 1.4K in triode according to Ale
6P21S – Bartola(R) Valves
It's not a valve I like the sound of personally, but whatever. Assuming that you'd use a resistor of at least 3x the Ri you would use something like 4.7K or preferably more. So you'd have around 70v or more across the resistor, which is quite realistic, as you say.
Thanks Rod, I took the grid stopper down to 220R. I could probably take it lower. So you are suggesting both a cathode bypass cap and another cap connecting the cathode to the HT at the top of the plate choke? Never tried that. What's the reasoning behind this?
I have the preamp up and running with 25mA going through the 6C4C. My target was 30mA so I can play about with the operating point. The Hammond 157G is a cheap and cheerful plate choke, 30H at 40mA. I should swap in a pair of my amorphous ones to see the difference. Smoother treble probably.
The sound is pretty much as before - very solid with good dynamics and a neutral and what sounds like a pretty even frequency response. It's more transparent than I expected - it doesn't change the sound too much. The treble isn't quite as silky, which is something I'd need to look at. But there are some benefits in dynamics and fullness and body of sound. There's something to be said for a beefy tube stage, even on the input.
Early days - I've only played a few tracks. But I think this has good potential. The output impedance is usefully low on a 6C4C so a plate choke can work - keep it all very simple.
You don't see a 300b or 2a3 used much as a preamp/line stage tube. Of the two I'd go for a 2a3 or a 6C4C I think, like I use in my outputs. It's not obvious why there aren't more line stages using these kind of tubes, or indeed other beefy tubes like 10Y, 45 and 46. Those don't have the low output impedance of the 6C4C, however.
I have the preamp up and running with 25mA going through the 6C4C. My target was 30mA so I can play about with the operating point. The Hammond 157G is a cheap and cheerful plate choke, 30H at 40mA. I should swap in a pair of my amorphous ones to see the difference. Smoother treble probably.
The sound is pretty much as before - very solid with good dynamics and a neutral and what sounds like a pretty even frequency response. It's more transparent than I expected - it doesn't change the sound too much. The treble isn't quite as silky, which is something I'd need to look at. But there are some benefits in dynamics and fullness and body of sound. There's something to be said for a beefy tube stage, even on the input.
Early days - I've only played a few tracks. But I think this has good potential. The output impedance is usefully low on a 6C4C so a plate choke can work - keep it all very simple.
You don't see a 300b or 2a3 used much as a preamp/line stage tube. Of the two I'd go for a 2a3 or a 6C4C I think, like I use in my outputs. It's not obvious why there aren't more line stages using these kind of tubes, or indeed other beefy tubes like 10Y, 45 and 46. Those don't have the low output impedance of the 6C4C, however.
Thanks for that. My coupling cap is 0.2uF, a pair of FT-2 caps. The bass seems to be there, though I have to confess I use a pair of bookshelf speakers.
I'm rather enjoying listening to this preamp. I don't need the extra gain, but the sound is fuller and more dynamic with it in the system.
Oh dear....... does this mean an end to my quiet life with just 2 stages after the DAC and back to a 3 stage DHT amp? All that extra work and complexity.....
I'm rather enjoying listening to this preamp. I don't need the extra gain, but the sound is fuller and more dynamic with it in the system.
Oh dear....... does this mean an end to my quiet life with just 2 stages after the DAC and back to a 3 stage DHT amp? All that extra work and complexity.....
After a week of listening I'm pretty sure that the 6C4C first stage is staying in my system for now. So all new for 2022. This does indeed mean a 3 stage DHT amp, which I've avoided for years, but it looks to be worth it. The first and last stages are 6C4C (or 2a3 outputs) - that much seems settled. So it's now a question of choosing the driver. Contenders are 10Y, 47, 46, maybe 2P29L. Something with at least 15mA through it. I'll put the first 2 stages on one chassis and the outputs in a separate chassis. Not sure how to organise the first 2 stages. I have outboard filament supplies with choke input, and outboard power supplies so I have to choose what goes in the box. The first 2 stages could be direct coupled possibly. I haven't done this with DHTs but since there's no cathode>heater issues and the filament supplies are floating I'm assuming it's straightforward?
Hi Andy,
Calculating of driver gain:
The 2A3 -for example- has 43V bias (B+:300V, 5k:8 OPT, 250V, 50mA op. point).
For reach A1 border (at about 2W output) the first stage must gain more than 15 (2VRMS input).
If you want 3W, the driver gain must be at least 19.
Even EML20/30 fulfills this, but another DHT tubes -without SUT- not.
p.s.
Let's get rid of my favourite large gain DHT tube 841/VT51, which practically inoperable -in A1 mode- with this low B+.
Calculating of driver gain:
The 2A3 -for example- has 43V bias (B+:300V, 5k:8 OPT, 250V, 50mA op. point).
For reach A1 border (at about 2W output) the first stage must gain more than 15 (2VRMS input).
If you want 3W, the driver gain must be at least 19.
Even EML20/30 fulfills this, but another DHT tubes -without SUT- not.
p.s.
Let's get rid of my favourite large gain DHT tube 841/VT51, which practically inoperable -in A1 mode- with this low B+.
> The first 2 stages could be direct coupled possibly. I haven't done this with DHTs but since there's no cathode>heater issues and the filament supplies are floating I'm assuming it's straightforward?
Usual DC coupling precautions: the first stage feeds DC to the grid of the second. But the DC voltage depends on the first stage anode current, so you can quickly run into trouble with high voltage on the grid, as the anode current drifts, getting higher as the filament ages.
DC coupling eats into the maximum anode swing on the driver stage, which is not helpful in a DHT power stage.
Also, we need a very slow ramp-up of the anode supply (as with 6CJ3 or similar damper diode), or face the risk of the DC coupled grid starting at high voltage.
I think it's a lot less trouble to find a few more FT-2 coupling caps!
Usual DC coupling precautions: the first stage feeds DC to the grid of the second. But the DC voltage depends on the first stage anode current, so you can quickly run into trouble with high voltage on the grid, as the anode current drifts, getting higher as the filament ages.
DC coupling eats into the maximum anode swing on the driver stage, which is not helpful in a DHT power stage.
Also, we need a very slow ramp-up of the anode supply (as with 6CJ3 or similar damper diode), or face the risk of the DC coupled grid starting at high voltage.
I think it's a lot less trouble to find a few more FT-2 coupling caps!