Has anyone compared the performance of the 12HG7 with the D3A.? Specs are similar - maybe the 12HG7 is a US copy of the D3A using similar techniques (a frame grid, for instance).
There is also a gaggle of pentodes with about half the transconductance of the D3A and 12HG7 used as IF and video amplifiers. They take about 1/2 the plate current of their high-transconductance siblings to make them wake up - not too surprising, I guess. One of them, the 6JD6, is listed in the ESRC dollar sale. I didn't get any, as I've been concentrating on the 6EW6, a tube with very similar specs. I've been looking at using tjhe 12HG7 as a one-stage driver for a screen driven sweep tube amp with 6P45S final stage. I plan to use the 6EW6 in an RH-style amp using the 6P3C-E (the tube touted as a KT88 substitute but really not up to the abuse).
For those who haven't seen it (I realize that I'm mostly preaching to the choir here), Peter Millett has a list of high transconductance pentodes at http://www.pmillett.com/ with test results for both triode and pentode mode that is very interesting reading. A lot of them are tubes that haven't achieved audiophool status yet, so they're still plentiful and inexpensive. Some of the IF-type pentodes I've mentioned aren't on that list, but I wouldn't be surprised if they performed as well as ones with similar specs that are there.
There is also a gaggle of pentodes with about half the transconductance of the D3A and 12HG7 used as IF and video amplifiers. They take about 1/2 the plate current of their high-transconductance siblings to make them wake up - not too surprising, I guess. One of them, the 6JD6, is listed in the ESRC dollar sale. I didn't get any, as I've been concentrating on the 6EW6, a tube with very similar specs. I've been looking at using tjhe 12HG7 as a one-stage driver for a screen driven sweep tube amp with 6P45S final stage. I plan to use the 6EW6 in an RH-style amp using the 6P3C-E (the tube touted as a KT88 substitute but really not up to the abuse).
For those who haven't seen it (I realize that I'm mostly preaching to the choir here), Peter Millett has a list of high transconductance pentodes at http://www.pmillett.com/ with test results for both triode and pentode mode that is very interesting reading. A lot of them are tubes that haven't achieved audiophool status yet, so they're still plentiful and inexpensive. Some of the IF-type pentodes I've mentioned aren't on that list, but I wouldn't be surprised if they performed as well as ones with similar specs that are there.
wrenchone said:
Your sweep tube idea sounds cool. I had been thinking about doing a screen driven PP amp, high gm pentode driving the dc coupled source followers and then using plate to plate feedback. I have a bunch of 6FW5's to try out for finals. I need a power supply or two and then I can start setting up a bread board amp to start testing.
What's an RH style amp?
I got a few of the cheap 6FW5's too. I'll be nice to those. The 6FW5 is an improved version of my old friend the 6AV5 and is pin compatible. They often go for stupid money on Ebay, but a tube vendor listed them for $3 a while back. They didn't last long. I got 10.
I have too many tubes already, but I couldn't resist this one. At the risk of making them all dissapear I'll mention this. There is one sweep tube on Stan's Dollar list, so I got 10 when I was in Orlando. Guess what guts are inside the glass?
6FW5 on the left. mystery tube on the right......
Have I got anyones attention? Number? OH, you want the number? It is the 6GV5. I am going to make some of these glow. How many watts can I get? It will be a few weeks before I find out.
It is the name given to a plate to plate feedback besign by member Alex Kitic. The circuit is similar to other plate to plate designs. His web site has the details. Lots of pop up adds send my firewall into overdrive, but they appear to be harmless.
http://www.tubeaudio.8m.com/
I went to a hamfest today and only bought ONE tube. The urge was strong, but I just said no!
Attachments
George, thanks for stepping in with the link to the RH amps. Mine will end up kinda-sorta like the 807 version, but with a pentode front end and a hatless output tube. I haven't tried a design without feedback around the output XFMR, so I'm curious whether the bass will be up to snuff. I'm using Edcor GXSE output transformers, and they're a little skimpy on primary inductance. I'm withstanding the temptation of buying yet more sweep tubes, even at $1 each, as I have lots already.
Compactrons are cute. Nice find.
I was the one responsible for taking all the 6FW5s. I was thinking of a monster amp when I bought the 30 something they had. Except I am not hoarding them and would share if someone was having a hard time finding some.
A screen driven PP amp with 6FW5s would be more than enough power for me I think. I was listening to Metallica earlier tonight with my KT88 e-linear on 90ish-db 4 ohm MTMs in the low 90's db ballpark and only at times were there hints of distortion. So I think near 80 watts would be sufficient.
I was the one responsible for taking all the 6FW5s. I was thinking of a monster amp when I bought the 30 something they had. Except I am not hoarding them and would share if someone was having a hard time finding some.
A screen driven PP amp with 6FW5s would be more than enough power for me I think. I was listening to Metallica earlier tonight with my KT88 e-linear on 90ish-db 4 ohm MTMs in the low 90's db ballpark and only at times were there hints of distortion. So I think near 80 watts would be sufficient.
Ideas on LED bias
Dear Doug,
Just got my board from my mailbox last night. Nice!
Maybe a silly question but please help if you can.
How it should be done if I want to use LED for bias?
From the plate curves, it seems I would need a bias voltage in the range of -10V to -12V
So I would need a string of 6 red LEDs or even a matrix of 6x6 red LEDs to obtain the bias?
Also can anybody post their experience on playing with D3a with this board?
Dear Doug,
Just got my board from my mailbox last night. Nice!
Maybe a silly question but please help if you can.
How it should be done if I want to use LED for bias?
From the plate curves, it seems I would need a bias voltage in the range of -10V to -12V
So I would need a string of 6 red LEDs or even a matrix of 6x6 red LEDs to obtain the bias?
Also can anybody post their experience on playing with D3a with this board?
Re: Ideas on LED bias
I think you looked at the wrong datasheet. From the plate curves, you want to bias between 1.5V and 1.75V.
http://www.mif.pg.gda.pl/homepages/frank/sheets/128/d/D3a.pdf
rtsang said:
I think you looked at the wrong datasheet. From the plate curves, you want to bias between 1.5V and 1.75V.
http://www.mif.pg.gda.pl/homepages/frank/sheets/128/d/D3a.pdf
Attachments
Re: Re: Ideas on LED bias
Hi Doug,
Yes, I have read the datasheets. But have you noticed that they put a +10V on the grid to generate the plate curves. If you calculate the voltage drop across the 470R cathode resistor, 20mA would cause a drop of 9.5V.
Correct me if I was wrong, the actual bias voltage from the plate curves should be -(10V + the grid curve voltage).
Raymond
dsavitsk said:
Hi Doug,
Yes, I have read the datasheets. But have you noticed that they put a +10V on the grid to generate the plate curves. If you calculate the voltage drop across the 470R cathode resistor, 20mA would cause a drop of 9.5V.
Correct me if I was wrong, the actual bias voltage from the plate curves should be -(10V + the grid curve voltage).
Raymond
You are, of course, welcome to do what you want. The datasheet is as it is because in the 1950's when the datasheets were written, big electrolytics, as would be necessary to bypass the small resistor necessary to create a bias of 1.5V at 15mA, were expensive and terrible. So, they biased the control grid positive such that Rk would be in the 1K ohm range rather than in the 100 ohm range.
However, this is a board meant for measurement, so you can build one with a +10V grid bias, an input cap, and a small Rk bypass and another without the input cap, a small Rk, and a larger Rk bypass and see what happens.
Maybe there is more to it and the + grid bias is beneficial in other ways, too.
However, this is a board meant for measurement, so you can build one with a +10V grid bias, an input cap, and a small Rk bypass and another without the input cap, a small Rk, and a larger Rk bypass and see what happens.
Maybe there is more to it and the + grid bias is beneficial in other ways, too.
pin/jumper part numbers are on the webpage: http://www.ecp.cc/d3a_exp.html
I don't know terminal blocks part #'s, but they are 5mm, or maybe 5.08mm (I don't remember but either will fit) and are easy to find by browsing through the catalogs.
I don't know terminal blocks part #'s, but they are 5mm, or maybe 5.08mm (I don't remember but either will fit) and are easy to find by browsing through the catalogs.
Hi, I am seriously questioning the maximum limit values of the D3a, trying to design a 150V 120Vpp DC coupled cathode follower. The bias is -1.75V, 11ma, 310 V HT, triode strapped (g2 + 100R + g3 +50R--- to anode).
Unfortunately it seems that I am exceeding the datasheet value of Vg2 max 180V as the AC voltage of the grid & cathode will fall to 90 V (150-60) causing a 220 V potential ... 40V over the limit.
(Reference at p.5 the Vg2o 400 V and Vg2 max is 180 V, http://www.hifitubes.nl/weblog/wp-content/philips-d3a.pdf)
Gabriel
Unfortunately it seems that I am exceeding the datasheet value of Vg2 max 180V as the AC voltage of the grid & cathode will fall to 90 V (150-60) causing a 220 V potential ... 40V over the limit.
(Reference at p.5 the Vg2o 400 V and Vg2 max is 180 V, http://www.hifitubes.nl/weblog/wp-content/philips-d3a.pdf)
Gabriel
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