Am currently spec'ing a interstage coupled 45 amp driven with a 26. I am aware of the hum and microphonic issues with the 26, however, I am keen in pursing a dht driving dht amp. Just wondering what are the some suggestions as to the operating point of the 26, and the method of bias that would be best. The 45 is operating at 50mA/250V (plate to filament, B+ of 300V)/-50V bias/5K load. Will be using a Tentlabs Autobias module along with the Tentlabs Filament supply.
The 26 will have a Tentlabs 1A low noise filament supply and coupled to the 45 via a 1:1 interstage. It looks like a 5K:5K unit, but might go for a custom impedance unit if alternative can be provided.
I'd assume that the IT will provide an almost equivalent choke loading of the 26.
Questions:
1. Can a 9V bias using a 9V rechargeable battery be good, or an alternate method of cathode bias?
2. What is a good plate voltage for the 26?
Thanks again!
The 26 will have a Tentlabs 1A low noise filament supply and coupled to the 45 via a 1:1 interstage. It looks like a 5K:5K unit, but might go for a custom impedance unit if alternative can be provided.
I'd assume that the IT will provide an almost equivalent choke loading of the 26.
Questions:
1. Can a 9V bias using a 9V rechargeable battery be good, or an alternate method of cathode bias?
2. What is a good plate voltage for the 26?
Thanks again!
eeyore said:
Not too sure how good a #26 is in that application. The #26 has a rather high r(p) which means that you'll need more L in the primary for good low frequency operation. Best to run it hot. Here's a suggestion from the spec sheet:
Vpk= 180Vdc
Vgk= -14.5Vdc
Ipq= 6.2mA
r(p)= 7K3 (Problematic: more modern types can do way better with similar u-factors.)
A 12B4 would be a helluvalot better: r(p)= 1K03
An externally hosted image should be here but it was not working when we last tested it.
So long as you keep the 45 out of Class *2 operation, the secondary disappears for all practical purposes.
1) No, you'll need more than 9.0Vdc. Your best bet is fixed bias for the #26. Since you're using an IT, cathode bias becomes problematic indeed. A cathode resistor bypass capacitor means capacitive currents in the cathode circuit, which means capacitive currents in the plate circuit, and that can cause resonances with the primary inductance. This will cause some nasty distortion. If you leave the cathode resistor unbypassed, that'll raise the effective r(p) and ruin your low frequency performance (gonna be difficult as it is, given the r(p)) besides, this puppy doesn't have the gain to spare.
Your best bet is fixed bias for the #26, with the cathode returned directly to ground.
My original intent was to us a 417A/5842 to drive the 45, however, was wanting to try something different and use a DHT instead. Are there alternate DHT that are more suitable? From searching, most low Rp DHT are the power tubes like 300B, 2A3, etc.
I do have some 12B4A so not adverse to this tube, what do you think would be a good operating point.
Doing a search for other single triode low Rp tubes; 6CK4, PC25, VR40. Higher mu tubes come up as CV5112, 3A/167M, E288CC, 8233, 7044, 7370, 8456 and 6BX7.
The other ones I have also considered at D3a, C3g/C3m, 6C45pi, etc.
What alternatives exist?
I do have some 12B4A so not adverse to this tube, what do you think would be a good operating point.
Doing a search for other single triode low Rp tubes; 6CK4, PC25, VR40. Higher mu tubes come up as CV5112, 3A/167M, E288CC, 8233, 7044, 7370, 8456 and 6BX7.
The other ones I have also considered at D3a, C3g/C3m, 6C45pi, etc.
What alternatives exist?
If you want to work in Class A1 then the 26 is ok and, IMO, much better than the options you listed above..... the tube is microphonic only if you fix it directly to the chassis, otherwise it is fine.
I would go for 275V/33-34 mA with 5K load for the 45.
Having approx. 270V for the 26 you could bias with 1.8K cathode resistor, 22K anode resistor. In this way the tube should work at 135V/5.5mA with -10V bias.
Then if you want to improve and get also some more power output you could bias the 45 with a direct coupled 6SN7 cathode follower (or mosfet source follower).
Cheers,
45
I would go for 275V/33-34 mA with 5K load for the 45.
Having approx. 270V for the 26 you could bias with 1.8K cathode resistor, 22K anode resistor. In this way the tube should work at 135V/5.5mA with -10V bias.
Then if you want to improve and get also some more power output you could bias the 45 with a direct coupled 6SN7 cathode follower (or mosfet source follower).
Cheers,
45
Actually, if you self-bias the 45 you will need 275+56 = 326V and this would mean a better supply for the 26.
In that case, using a 2.2K cathode resistor and 22K anode resistor, the 26 would work at approx. 180V/6mA. Goal!!
In this case you should get approx. 2W, while with the previous solution 1.5W only.
Cheers,
45
In that case, using a 2.2K cathode resistor and 22K anode resistor, the 26 would work at approx. 180V/6mA. Goal!!
In this case you should get approx. 2W, while with the previous solution 1.5W only.
Cheers,
45
I've used as drivers 12B4, 6CK4, 6AH4, 1626 etc etc and nothing is as good as a nice DHT like the 26. I've used that with a 1:1 interstage. The LL1660 is one possibility. As said, if you have the voltage, go for somewhere near 180v. Not over that.
Alternatives are 12A - similar operating points and less greedy on the filament, but 26 sounds better. 01A is a very nice sound but max plate voltage here is 135v and quoted as 90v for some of the older globes. 10Y is another very good choice. All these are excellent, and have about a mu of around 9. 71A and 31 sound nice but lower gain. 1J6g is mu of 15 and sounds good - that's a dual triode, common cathode. I use it in PP - never tried the sections in parallel, but could be good.
I see nothing wrong with going ahead with the 26. Others have used it with excellent results. It's a great tube. I used cathode bias with a polypropylene bypass (wouldn't use an electrolytic).
andy
Alternatives are 12A - similar operating points and less greedy on the filament, but 26 sounds better. 01A is a very nice sound but max plate voltage here is 135v and quoted as 90v for some of the older globes. 10Y is another very good choice. All these are excellent, and have about a mu of around 9. 71A and 31 sound nice but lower gain. 1J6g is mu of 15 and sounds good - that's a dual triode, common cathode. I use it in PP - never tried the sections in parallel, but could be good.
I see nothing wrong with going ahead with the 26. Others have used it with excellent results. It's a great tube. I used cathode bias with a polypropylene bypass (wouldn't use an electrolytic).
andy
You could get really wild and build Gary Pimm's All C4S Loaded Parafeed 45 which uses the 26 as a driver. http://www.pacifier.com/~gpimm/45_all_active.gif
James
James
eeyore said:
The cathode resistor for the 26 should be around 2.2K. You could start with 1K resistor + 1.5K pot.....
If you get 180V by means of RC decoupling and filtering from the 300V, then I would go for 200V (because of 13-14V bias + some drop at the primary of the IT transformer).
If you want to use batteries you could try with 3 x 4.5V volt common alkaline batteries, all in series with the 47K grid resistor.
Like this: http://www.watchbattery.co.uk/shop/products/BADR-3LR12.shtml
This has been done and works very well for long long time, in that way.
Cheers,
45
eeyore said:
This is not the case because the cathode is grounded and the battery is between the ground and the grid (with the 47K resistor between them): it is exactcly as any conventional fixed bias from this point of view.
Maybe the battery can introduce a ridicoulus (due to its very small internal resistance) amount of feedback if you place it between the cathode and the ground. In this case it will not last for a long time, too.
However, the LED as well has a small series resitance, too!
I would be much more worried about the IT transformer which has to have a quite high primary inductance (at least 120-130H with 6 mA DC), extended frequency response (with small phase shift) and low distortion. Not easy to get one like this....
Cheers,
45
For 45 tubes a little bit other driver's tube is necessary.
For two reasons.
For fullpower (rock or modern record) music the current in 6 mA all the same is not enough for a grid 45. Value of a current will be on an upper limit for 45 tubes. It not to eat well.
For achievement of passport power in two watts the input effective voltage 7.89 V is required to you. On grid of the first tube (26), with IT1:1 and its input impedance 10 kOhm.
BW, VU
For two reasons.
For fullpower (rock or modern record) music the current in 6 mA all the same is not enough for a grid 45. Value of a current will be on an upper limit for 45 tubes. It not to eat well.
For achievement of passport power in two watts the input effective voltage 7.89 V is required to you. On grid of the first tube (26), with IT1:1 and its input impedance 10 kOhm.
BW, VU
eeyore said:
For just 2W output it looks exaggerate to me. Moreover, even with such combination you still need an excellent pre-amp!
In my experience too many transformers = questionable results......
Personally, I would go for a 3-stage integrated amplifier, using a medium mu double-triode. In that case you could use an RC coupling to the 26 which is what you need for the battery fixed bias.
I would invest my money in the power supply: one supply for the input and 26's stages, one supply for the 45's and one supply for the 45's bias. Filaments: all DC supplied.
45
P.S.
Medium mu = 20 - 40.
eeyore said:
Between CCS and choke I would go for the CCS.
You can use CCS for the input so that it will be distortion free at any modulation level of the amp.
Then I would use resistive loading for the 26 together with a DC-coupled cathode (or source) follower to bias and drive the 45.
Having an independent supply for the 26, you could set up its operative condition as you like, for example, using 380V/33K to 320V/22K for supply voltage and anode resistor, respectively.
A good 26 will have no distortion at low to medium level and dominant 2nd harmonic at high level.
Its THD is of the same type of the 45 (This is a very important point!).
So, approaching the max output you could get benefit from some harmonic cancellation being out of phase between them...
This is basically what I am experimenting for my next amp (which is, however, a little more complicated PP) and it appears to work (sound) very well.
Cheers,
45
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.