Dear Niko, don't take all the worry about tubes but about output transformer.
What are the budget?
Can you afford a pair of MONOLITH or Lundahl ?
Thimios.
Please use the group buy thread linked below to place your orders and conduct all discussions pertaining to the PCB.GB for Baby Huey PCB
A question on power transformer choice
According to the post below a toroidal power transformer with 230V secondaries will result in ~302V plate voltage.
http://www.diyaudio.com/forums/tubes-valves/72536-el84-amp-baby-huey-167.html#post4848377
Multiplying 230 x 1.4 = 322V, so I am guessing it will be in this range.
With the plate voltage around 300-320V, what tubes will be ideal?
I am looking at 6V6 and also at EL34/6CA7... with 300V on plate, is it too low for the EL34 / 6CA7?
According to the post below a toroidal power transformer with 230V secondaries will result in ~302V plate voltage.
http://www.diyaudio.com/forums/tubes-valves/72536-el84-amp-baby-huey-167.html#post4848377
Multiplying 230 x 1.4 = 322V, so I am guessing it will be in this range.
With the plate voltage around 300-320V, what tubes will be ideal?
I am looking at 6V6 and also at EL34/6CA7... with 300V on plate, is it too low for the EL34 / 6CA7?
Do you have the power transformers and output transformers already? Or will you be purchasing those?
If you want to use EL84 or 6V6 type output tubes, the output transformers should probably have primary impedance of 8k to 10k.
If you want to use EL34 or 6L6 type output tubes, the output transformers should probably have primary impedance of 5k to 6.6k. These tubes draw more plate current, so will need bigger power transformer(s) too. With plate voltage of about 300 to 320V, figure on each EL34 or 6L6 drawing 60mA or more for good results.
If you want to use EL84 or 6V6 type output tubes, the output transformers should probably have primary impedance of 8k to 10k.
If you want to use EL34 or 6L6 type output tubes, the output transformers should probably have primary impedance of 5k to 6.6k. These tubes draw more plate current, so will need bigger power transformer(s) too. With plate voltage of about 300 to 320V, figure on each EL34 or 6L6 drawing 60mA or more for good results.
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Hi zman01,
Don't forget about the higher heater currents! EL34's can suck back 1.4 amperes each, the 6L6GC would be about 900mA each. The EL84/6BQ5 wants 760mA for heater current.
It would seem to be a huge waste of energy using EL34's with only a 300V B+. From the looks of things, it makes little difference in heater current whether you go with the 6L6GC or EL84/6BQ5. The plate current would then be the only concern between them. Of course you would need to make up your mind before buying output transformers (or the 6L6GC transformer could run a quad of 6BQ5 tubes). The cost for the quad vs a pair of 6L6GC is pretty close to the same money too. If it were me, I would only use one pair of outputs per channel.
-Chris
Don't forget about the higher heater currents! EL34's can suck back 1.4 amperes each, the 6L6GC would be about 900mA each. The EL84/6BQ5 wants 760mA for heater current.
It would seem to be a huge waste of energy using EL34's with only a 300V B+. From the looks of things, it makes little difference in heater current whether you go with the 6L6GC or EL84/6BQ5. The plate current would then be the only concern between them. Of course you would need to make up your mind before buying output transformers (or the 6L6GC transformer could run a quad of 6BQ5 tubes). The cost for the quad vs a pair of 6L6GC is pretty close to the same money too. If it were me, I would only use one pair of outputs per channel.
-Chris
This kind of local plate-grid feedback has been shown to work really well for 6L6 tubes.
There are a few folks around here who've made more elaborate designs using type 807 for the output tubes, with pentode drivers. 807 is basically a 6L6GB with a plate cap for higher plate voltage capability.
Looking at load lines, it seems that if you run a 6V6 at 300V on both the plate and screen, and about 45mA plate+screen current, the grid bias needs to be about -20V.
Looking at a 6L6 run at the same plate and screen voltage but with 65mA plate+screen current, it needs a grid bias of about -21V.
So they require about the same fairly low voltage swing from the 12AX7+source follower drivers. Food for thought...
The advantage of using EL84 is that at 300V plate+screen and 40mA (grid bias of -12V) it will require only about 12V peak from the driver stage. That's a big difference. EL84 is just about the easiest to drive output tube there is.
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There are a few folks around here who've made more elaborate designs using type 807 for the output tubes, with pentode drivers. 807 is basically a 6L6GB with a plate cap for higher plate voltage capability.
Looking at load lines, it seems that if you run a 6V6 at 300V on both the plate and screen, and about 45mA plate+screen current, the grid bias needs to be about -20V.
Looking at a 6L6 run at the same plate and screen voltage but with 65mA plate+screen current, it needs a grid bias of about -21V.
So they require about the same fairly low voltage swing from the 12AX7+source follower drivers. Food for thought...
The advantage of using EL84 is that at 300V plate+screen and 40mA (grid bias of -12V) it will require only about 12V peak from the driver stage. That's a big difference. EL84 is just about the easiest to drive output tube there is.
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Chris, rongon,
Thank you for your replies and advice.
I haven't bought any iron yet, so have my options open. It is easier (and cheaper) for me to get a toroidal transformer with 230V secondaries (400 mA spec), so would like to go for that and choose other components accordingly. With 400 mA, the current rating should be sufficient for 4 x EL34 / 6CA7 tubes. However as Chris pointed out, a 300V B+ is low for an EL34 type tube.
If I go for the EL34 I shall be choosing a 6.6K OPT, and with this setup I shall be able to try out 6L6, 6CA7 and possibly KT66 tubes too. If I go for the 6V6, then I'll have to choose a 8K OPT and that will limit the potential of rolling tubes (tube rolling option is nice to have, but not a must
).
For the filaments I plan to use a separate regulated power supply capable of 6.3V and rated at 10 amps. Btw, I have signed up for the PCB GB and plan to use the boards for the build.
With 300V B+ and EL34 / 6CA7 tubes, I understand that I shall be throwing away a few watts (due to lower B+), but do you think it will result in worse sound (considering THD value, profile at that operating point of plate voltage)? My priority would be sound quality.
Thank you for your replies and advice.
I haven't bought any iron yet, so have my options open. It is easier (and cheaper) for me to get a toroidal transformer with 230V secondaries (400 mA spec), so would like to go for that and choose other components accordingly. With 400 mA, the current rating should be sufficient for 4 x EL34 / 6CA7 tubes. However as Chris pointed out, a 300V B+ is low for an EL34 type tube.
If I go for the EL34 I shall be choosing a 6.6K OPT, and with this setup I shall be able to try out 6L6, 6CA7 and possibly KT66 tubes too. If I go for the 6V6, then I'll have to choose a 8K OPT and that will limit the potential of rolling tubes (tube rolling option is nice to have, but not a must
). For the filaments I plan to use a separate regulated power supply capable of 6.3V and rated at 10 amps. Btw, I have signed up for the PCB GB and plan to use the boards for the build.
With 300V B+ and EL34 / 6CA7 tubes, I understand that I shall be throwing away a few watts (due to lower B+), but do you think it will result in worse sound (considering THD value, profile at that operating point of plate voltage)? My priority would be sound quality.
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Andrew,
Have read that Electra-Print transformers are very good. I would have also loved to try the 6SL7, but the PCBs in a recent Group Buy done by diyA member bandol83 are for 12AX7.
10 good watts go a long way.
Have read that Electra-Print transformers are very good. I would have also loved to try the 6SL7, but the PCBs in a recent Group Buy done by diyA member bandol83 are for 12AX7.
10 good watts go a long way.
I'm looking forward to getting a couple of those boards too. I'm planning to use 6L6GB tubes in mine (I have a bunch of those), with a pair of 5k output transformers I have on hand. I also have a few 500VCT 150mA power transformers, which should give me about +325V DC using UF4007 rectifiers. Subtracting -21V from that, I'm looking at right about +300V plate+screen to cathode for the 6L6s.
Modeling this assemblage in LTspice, I get predicted full power output of about 17 watts. If I use 6V6s and an 8k:VC OPT instead, max power out is about 10 watts (just like in real life, apparently ).
Modeling this assemblage in LTspice, I get predicted full power output of about 17 watts. If I use 6V6s and an 8k:VC OPT instead, max power out is about 10 watts (just like in real life, apparently
).Hi,
With the first version that I have built, I have used a 250 V AC toroïdal transformer giving giving me about 350 V DC (the main line is about 244 V AC instead of 230 V here!) I have tested 6V6S tubes from JJ with both a 8k 15W TU-101 copy and an Hammond 1609 10k 10W transformer. The JJ tubes are specified at higher voltage than NOS tubes, they were very good in this setup.
Before I had tested old EL34 not very high quality, there was no more indication on the tube so I cannot really comment, this setup was really made to check that everything was OK with the PCB and I didn't want to destroy expensive new tubes in case of error
After I have tested 6CA7 JJ tubes again with the same Hammond 1650N 4,3k 60W output transformer that I have used for the EL34 and, for me, that is the best version until now. I have also tested one channel (I didn't have enough tubes...) with the KT88, but I didn't like it, may be this tube need more than 350 V ?
I read that some people give a very positive criticism for the KT66 but I have not tested them yet, may be they require a 6,6k transformer like the 6L6 ?
All the test were made with the same power supply transformer and the amplifier was very stable in all case without global feedback.
What is interesting with this amplifier, is that you can try many different output tubes very easily
Regarding a previous comment, I confirm that you need nearly 7A for the heaters of a 6CA7 stereo amplifier! I am using a small 6.3 V 10A SMPS.
Best regards and good night,
Marc
With the first version that I have built, I have used a 250 V AC toroïdal transformer giving giving me about 350 V DC (the main line is about 244 V AC instead of 230 V here!) I have tested 6V6S tubes from JJ with both a 8k 15W TU-101 copy and an Hammond 1609 10k 10W transformer. The JJ tubes are specified at higher voltage than NOS tubes, they were very good in this setup.
Before I had tested old EL34 not very high quality, there was no more indication on the tube so I cannot really comment, this setup was really made to check that everything was OK with the PCB and I didn't want to destroy expensive new tubes in case of error
After I have tested 6CA7 JJ tubes again with the same Hammond 1650N 4,3k 60W output transformer that I have used for the EL34 and, for me, that is the best version until now. I have also tested one channel (I didn't have enough tubes...) with the KT88, but I didn't like it, may be this tube need more than 350 V ?
I read that some people give a very positive criticism for the KT66 but I have not tested them yet, may be they require a 6,6k transformer like the 6L6 ?
All the test were made with the same power supply transformer and the amplifier was very stable in all case without global feedback.
What is interesting with this amplifier, is that you can try many different output tubes very easily
Regarding a previous comment, I confirm that you need nearly 7A for the heaters of a 6CA7 stereo amplifier! I am using a small 6.3 V 10A SMPS.
Best regards and good night,
Marc
Hi zman01,
The 6BQ5A is a really nice sounding tube. In the EL34 family, the 6CA7 sounds better in my view. The 6CA7 is a beam power tube - so more efficient than the EL34 which is a true Pentode (although you can get EL34 in the 6CA7 bottle marked as EL34 - they aren't).
6L6GC is a great sounding tube, but you're heading for higher power levels now (with the EL34 - 25 watt plate). You might also consider the 7591A tube as it is easier to drive than most.
-Chris
The 6BQ5A is a really nice sounding tube. In the EL34 family, the 6CA7 sounds better in my view. The 6CA7 is a beam power tube - so more efficient than the EL34 which is a true Pentode (although you can get EL34 in the 6CA7 bottle marked as EL34 - they aren't).
6L6GC is a great sounding tube, but you're heading for higher power levels now (with the EL34 - 25 watt plate). You might also consider the 7591A tube as it is easier to drive than most.
-Chris
If you try to use the full plate dissipation of a pair of 7581 or KT66 tubes, you'll need seriously big power and output transformers.
To reach 35 watts of plate dissipation with a plate-cathode voltage of +300V, each 7581 or KT66 would need to be run with 115mA of plate current. That's HOT.
25 to 30 watt output transformers aren't rated for that much standing plate current. If you go down this path, you'll want to use 60+ watt OPTs. Big and more expensive.
6L6GC has max plate dissipation of 30 watts. With +300V plate-cathode that would still be 100mA of plate current per tube.
Using 6L6GC, 7581A or KT66 at lower plate dissipation (say about 20 to 25 watts per tube) should help to attain long service life for the tubes.
6L6G, 6L6GA, 6L6GB, and 5881 have max plate dissipation of 20 watts.
A 6L6GB (or 5881) with 20 watts max plate dissipation could be run with Vp-k of 300V, Ip of 65mA (19.5W plate dissipation). That's within the limits of most 30 watt-rated output transformers. For example, the data sheet for the Tango XE-45-5 OPT says it's rated for 70mA on each primary winding, even though it claims a rating of 45 watts.
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To reach 35 watts of plate dissipation with a plate-cathode voltage of +300V, each 7581 or KT66 would need to be run with 115mA of plate current. That's HOT.
25 to 30 watt output transformers aren't rated for that much standing plate current. If you go down this path, you'll want to use 60+ watt OPTs. Big and more expensive.
6L6GC has max plate dissipation of 30 watts. With +300V plate-cathode that would still be 100mA of plate current per tube.
Using 6L6GC, 7581A or KT66 at lower plate dissipation (say about 20 to 25 watts per tube) should help to attain long service life for the tubes.
6L6G, 6L6GA, 6L6GB, and 5881 have max plate dissipation of 20 watts.
A 6L6GB (or 5881) with 20 watts max plate dissipation could be run with Vp-k of 300V, Ip of 65mA (19.5W plate dissipation). That's within the limits of most 30 watt-rated output transformers. For example, the data sheet for the Tango XE-45-5 OPT says it's rated for 70mA on each primary winding, even though it claims a rating of 45 watts.
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