Albert, from #1627: please make
R1 = 0.5
R2 = 1.5 + 25mH, series connected.
Choke in R1 position will reduce voltage. (If you have a 20V transformer, inductor here works well)
R1 = 0.5
R2 = 1.5 + 25mH, series connected.
Choke in R1 position will reduce voltage. (If you have a 20V transformer, inductor here works well)
Hi Andy,
I have and with great results. I tried 30, 30sp, 112,01a,10Y, 46, 4P1L, 6P21S in addition to 26. If you use the Rod Coleman filament regulators you can reduce the filament current precisely whilst measuring THD.
See some of my comments here:
http://www.diyaudio.com/forums/tubes-valves/190857-4p1l-dht-line-stage-17.html
I found effective not only to reduce THD but also microphonics, as this was the suggestion to address that issue on the 4P1L preamp.
When using triode strapped DHT, is interesting to find that there is a point where starvations magnifies the H3 and H5 above H2.
Currently I'm playing a 71a preamp with filament bias and gyrator loaded. I actually like it very much:
My Blog
Distortion levels can be reduced from 0.04-0.06% to 0.01-0.02% depending the valve 🙂
I'm currently experimenting with the 6P21S:
My Blog
Cheers,
Ale
Mogliaa did you find a decrease in transductance or increase in Ra with the reduced filament current on the 4p1l?
Hi Andy,
Can't find my notes and will need to measure it again but I remember it was around 60-70% (650-700 ma) of the data sheet value, so close to the values you posted before.
I spent quite a lot trying other valves so will need to come back to the 26 to refine it 😀
cheers,
Ale
Hi Regal,
Haven't measure both, so can't say. I focused on minimising microphonics and THD, which did with excellent results...
cheers,
Ale
Well I imagine if the RA went up much you would have seen a drop in the frequency response if you used a choke or output tranny, but I remembered you were using CCS OTL for the test setup.
I'd recommend others using an inductive load to check this by doing a freq response sweep if running lower currents. I ended up shelving the pair of #26's I had and bought a few 4P1L's, so as usual I won't be any help.
Mouser has 100 watt 10 ohm wirewound resistors for about 8 bucks. They also have thick film 100 watters for bolting to heatsinks for about the same price. Which would be a better choice for filament bias for the 26? Does the inductance of the wirewound have an impact on the circuit?
For $8 it's worth a try. With so much power to burn, you can sometimes get lower noise by using multiple parallel parts (eg 10x 100R). This will surely help reduce the noise contribution from the wire-attach junction, which plays a major part in the sound of resistors, I suspect. Andy has some recommended types, all wirewounds I believe. Parallel pairs of Welwyn W24 (vitreous enamelled wirewound 14W) are the minimum standard, cheap enough to use as a reference.
The thick film types can sound terrible - not recommended.
Inductance will certainly not upset the Filament Regulators. It may make the circuit more inclined to pick up EMI though, and this is potentially a serious problem. Keeping the wiring in tight loops will help.
The thick film types can sound terrible - not recommended.
Inductance will certainly not upset the Filament Regulators. It may make the circuit more inclined to pick up EMI though, and this is potentially a serious problem. Keeping the wiring in tight loops will help.
I just tried out some of these Russian types as cathode resistors (other values, same type)
90 x 2 Ohm 10W 2% Aluminium Wire Wound Resistors / NOS | eBay
They seemed to sound better than the Welwyn types, so that's good. The 5cm long ones are 10 watts. Otherwise I've been using Dale wirewound surplus stock from the USA which are great sounding.
eBay - The UK's Online Marketplace
Welwyn is a pretty good option.
Agree thick films are dreadful - worst of all. Ceramic are middle of the road, aluminium clad a little better but not great.
90 x 2 Ohm 10W 2% Aluminium Wire Wound Resistors / NOS | eBay
They seemed to sound better than the Welwyn types, so that's good. The 5cm long ones are 10 watts. Otherwise I've been using Dale wirewound surplus stock from the USA which are great sounding.
eBay - The UK's Online Marketplace
Welwyn is a pretty good option.
Agree thick films are dreadful - worst of all. Ceramic are middle of the road, aluminium clad a little better but not great.
I have my raw supply firing up and have the following number without load. Please do comment if the voltage is at the spot.
B+ 237v
Filament 1. 25.11v
Filament 2. 25.12v
Albert
B+ 237v
Filament 1. 25.11v
Filament 2. 25.12v
Albert
Hi Albert, I think these look OK.
For the Filament, you can connect the dummy load of 1.5-ohm and the 10-ohm bias resistor (in series).
Please check that you have 0.5R [R1] in the rectifier line, and total 2-ohm [R2+L1] in the C1 to C2 line.
Make sure you have a heatsink on the Regulator.
With the dummy load connected, expect about 17 to 19V of raw dc. Adjust the regulator's trimmer to get 1.5V across the 1.5R - should correspond to 11.5V across the output of the regulator.
when this adjustment is correct, check the raw dc at C2 again. Should be 16.5V to 17.5V. If it is more than 17.7V, then add some resistance to R2 to lower it.
For the Filament, you can connect the dummy load of 1.5-ohm and the 10-ohm bias resistor (in series).
Please check that you have 0.5R [R1] in the rectifier line, and total 2-ohm [R2+L1] in the C1 to C2 line.
Make sure you have a heatsink on the Regulator.
With the dummy load connected, expect about 17 to 19V of raw dc. Adjust the regulator's trimmer to get 1.5V across the 1.5R - should correspond to 11.5V across the output of the regulator.
when this adjustment is correct, check the raw dc at C2 again. Should be 16.5V to 17.5V. If it is more than 17.7V, then add some resistance to R2 to lower it.
Rod,
Thanks for the fast reply.
I'm now in the middle of reforming the main chassis for the reason that I don't have room for sinking the Sala's reg. I thought I have to place the 1660 at inside the chassis. Sala's reg., big heatsink, 26 & resistors will be on the top.
Will report asap
Albert
Input transformer?
Hello!
I just built a 26 preamp with 5:1 Audionote transformers and the sound is simply amazing. It smokes the 6sn7 or 5687 setup with the same output transformers on every level.
I want to go all DHT on my 300B monoblocks and have some 46 tubes that I will use as a driver. But what to choose as input tube? I probably also need 1:2 input transformers to get enough gain so can somebody give me any clues on what kind of specifications I must look for? Are the Audionotes any good?
Vidar
Hello!
I just built a 26 preamp with 5:1 Audionote transformers and the sound is simply amazing. It smokes the 6sn7 or 5687 setup with the same output transformers on every level.
I want to go all DHT on my 300B monoblocks and have some 46 tubes that I will use as a driver. But what to choose as input tube? I probably also need 1:2 input transformers to get enough gain so can somebody give me any clues on what kind of specifications I must look for? Are the Audionotes any good?
Vidar
I use 26 with plate choke into 46 (battery grid bias) into a Hammond 126C with end bells taken off into a 300B. Very good sound and enough volume since it's all 1:1 ratios.
Input tubes? another 26 or a VT67. But do you really want 4 stages? For a bit more gain, maybe 4P1L for the 46?
Andy
Input tubes? another 26 or a VT67. But do you really want 4 stages? For a bit more gain, maybe 4P1L for the 46?
Andy
Thank you for the response.
The preamp with 5:1 transformers is just to (damn) good to change at the moment so I need three stages on the monoblocks. Never heard of VT67. I´ll check it up.
Is there somebody who has recommendations for input transformers?
Vidar
The preamp with 5:1 transformers is just to (damn) good to change at the moment so I need three stages on the monoblocks. Never heard of VT67. I´ll check it up.
Is there somebody who has recommendations for input transformers?
Vidar
26 is up running
Hi,
I have my 26 running and seems quite healthy.
Raw filament after rectifier, I have 0.5 ohm, 470uf, 25mH + 2 ohm, 30,000uf - I have 17.6v on this point, but the 2 ohm 10w is hot at 58c after 3 hour, then to Rod's reg. 11.5v n 10v at 10 ohm. This 10 ohm is a group of 10 100 ohm 5w dale wirewound, the temperature within these 10 are vary from 75c to 105c after 3 hours. This is what I'm a bit worry about. Is there any way I could even out the temperature ??
On Hv, I have 182v to Sala's reg. and 150v supply to 26, 145v on plate of 26. Since I have a very big heatsink for the Sala's reg. so the temperature is only around 30c after 3 hour. The square wave didn't look too good and will do some experiment this couple days.
Will upload some pics later.
Albert
Hi,
I have my 26 running and seems quite healthy.
Raw filament after rectifier, I have 0.5 ohm, 470uf, 25mH + 2 ohm, 30,000uf - I have 17.6v on this point, but the 2 ohm 10w is hot at 58c after 3 hour, then to Rod's reg. 11.5v n 10v at 10 ohm. This 10 ohm is a group of 10 100 ohm 5w dale wirewound, the temperature within these 10 are vary from 75c to 105c after 3 hours. This is what I'm a bit worry about. Is there any way I could even out the temperature ??
On Hv, I have 182v to Sala's reg. and 150v supply to 26, 145v on plate of 26. Since I have a very big heatsink for the Sala's reg. so the temperature is only around 30c after 3 hour. The square wave didn't look too good and will do some experiment this couple days.
Will upload some pics later.
Albert