To throw another option out there, the PL84/EL86/6CW5 will do 25 watts into a 3k load, according to the datasheet. They need lower (150-175~) screen voltage, but are an absolutely fantastic tube, and the Russian equivalent 6P43P-E can be had very cheap. It's not unusual to push 30 watts or more through a pair without stressing the tubes. Takes 42~ volts or so grid to grid of drive voltage at 250 volts anode, 150-175 screen.
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It would be good to know if Antek can provide a model with similar specs. Please let us know.
The limitations for the 6V6 variant were addressed later by adding in the MOSFET Source Followers. This is the circuit for which Bandol has done the boards. By 6AC7 I assume you mean 6CA7. They are a good choice too. In fact I think someone (maybe even Bandol) mentioned that they liked the amp with 6CA7.
A Quad EL84 is indeed a workable option. You will note that I said that higher gm output tubes are theoretically the best for a BH. When you parallel output tubes you effectively get a "composite" tube of double the gm. Should work realy well. The design change I would do for the circuit at post #604?? would be to simple double up the source followers too, so each output tube has its own source follower.That would allow bias to be set for each tube individually.
For EL34 or 6CA7 they tolerate the larger max Grid 1 resistance and so you could use the simpler circuit way back on page 1 with the current source biasing.
I changed to fixed bias (with the source followers) to accommodate the lower max grid resistance requirements of tubes like 6V6, 6550, KT88 etc. but also because the current source biasing has "crap" overload recovery. OK if you don't drive the amp hard.
Cheers,
Ian
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Thanks I think my build options are coming into much better focus now. What is a conservative power output rating for a quad of el84 per channel in the configuration you describe above? Will I have to cut the OT’s value in half with the quad of tubes as in a PPP config? I.e. 4.3k / 2 = ~2.15k? What would you suggest for power and output tx values for this?
Yes it looks like #604 was your final, #602 was "spoke too soon" version, and #603 was the PSU.
schematics from #604 and #603 attached for reference.
You say the PSU was not optimized, using transformers already on hand. What differences would you suggest for a scratch build?
PS
Has anyone drawn this up on a computer? LTSpice or Kicad files?
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A quad of EL84 will give 20 Watts.
Use Output tansformer with Raa = 4K (1/2 the 8K that ypu would use for a pair). There are some common options at 4K2 and 4k3 either would do fine, 5K would also work.
Suitable examples: Hammond 1645A (5K 30W), Hammond 1650NA (4K3 60W).
You want a power tranny with around 240 - 260V AC secondary. I would be tempted to use an "off the shelf" 240V-250 V AC Output Isolation Transformer for the High Voltage, 100 to 120VA (monoblock) or 200 - 250VA for stereo.
Use separate 6V3 transformer for the heaters 25VA for monoblock or 50VA for stereo .
If you can get a ual secondary heater tranny then use one secindary for each channel.
Cheers,
Ian
Use Output tansformer with Raa = 4K (1/2 the 8K that ypu would use for a pair). There are some common options at 4K2 and 4k3 either would do fine, 5K would also work.
Suitable examples: Hammond 1645A (5K 30W), Hammond 1650NA (4K3 60W).
You want a power tranny with around 240 - 260V AC secondary. I would be tempted to use an "off the shelf" 240V-250 V AC Output Isolation Transformer for the High Voltage, 100 to 120VA (monoblock) or 200 - 250VA for stereo.
Use separate 6V3 transformer for the heaters 25VA for monoblock or 50VA for stereo .
If you can get a ual secondary heater tranny then use one secindary for each channel.
Cheers,
Ian
Hello Ian,
Nice to hear from you
I have worked on a quad EL84 before to decide to use the EL34, I have abandoned the quad output tube project because for a stereo amplifier it needed 8 source followers with 8 trimmers to adjust the bias of each tubes At a certain point I was thinking to use Norman Koren auto bias, but finally I found much more easy to replace the EL84 noval socket with an octal one to accommodate a wide choice of output tube from the 6V6 to the KT88...
Thanks again for your support, it worked very well and had a lot of success on this forum.
As you wrote, I have effectively said that I preferred the 6CA7 compared the KT88, I also tested the 6V6 with a Hammond 1609 10 k output transformer and the result was good but with the same power than the EL84 version, about 10 W. I didn't have KT77 in stock therefor I could not test them, but I hear some good criticism from others user's about this tube, it is interesting to see that the Baby Huey schematic is flexible enough to support different tubes without stability problems ! I have never user the global feedback input and the amplifier was always very stable I have added two lists of possible tubes with their main characteristics.
Back to the power transformer, the TOROIDY TSTA 250/001 will be perfect for a stereo amplifier, I have used a custom 141 VA transformer without problem in my setup (see photo), but for those who prefer a separate 6.3 V DC power supply I recommend to use a switching one which is availble for very reasonable cost and will not require a transformer.
Cheers,
Marc
Nice to hear from you
I have worked on a quad EL84 before to decide to use the EL34, I have abandoned the quad output tube project because for a stereo amplifier it needed 8 source followers with 8 trimmers to adjust the bias of each tubes
At a certain point I was thinking to use Norman Koren auto bias, but finally I found much more easy to replace the EL84 noval socket with an octal one to accommodate a wide choice of output tube from the 6V6 to the KT88...Thanks again for your support, it worked very well and had a lot of success on this forum.
As you wrote, I have effectively said that I preferred the 6CA7 compared the KT88, I also tested the 6V6 with a Hammond 1609 10 k output transformer and the result was good but with the same power than the EL84 version, about 10 W. I didn't have KT77 in stock therefor I could not test them, but I hear some good criticism from others user's about this tube, it is interesting to see that the Baby Huey schematic is flexible enough to support different tubes without stability problems ! I have never user the global feedback input and the amplifier was always very stable
I have added two lists of possible tubes with their main characteristics.Back to the power transformer, the TOROIDY TSTA 250/001 will be perfect for a stereo amplifier, I have used a custom 141 VA transformer without problem in my setup (see photo), but for those who prefer a separate 6.3 V DC power supply I recommend to use a switching one which is availble for very reasonable cost and will not require a transformer.
Cheers,
Marc
Attachments
A follow-up to the TOROIDY TSTA 250/001, this company, TSTA 250/001 TOROIDY - Transformer: toroidal audio | TME - Electronic components shipping to the US is approximately $10, which isn't too bad considering it is shipping from Poland or an EU warehouse.
I just spoke to Antek and they are reviewing the specs of this PT to determine doing a special run and should know something by weeks end.
I just spoke to Antek and they are reviewing the specs of this PT to determine doing a special run and should know something by weeks end.
Asking for a center tapped HV would be an improvement over the Toroidy as well.
Hi Ian,
Long time, good to "see" you.
Have you considered the 7591EH? That should give spaceistheplace exactly what he wants. Drive requirements aren't that much more than the 6BQ5 either.
Hi spaceistheplace,
For tube types, Electroharmonix from New Sensor are about the best new tubes, and they don't cost a lot either. Skip all the silliness of NOS types and just buy a set of new tubes. I find the Electroharmonix tubes are very consistent from tube to tube, and they also match what a good NOS tube measures. You can't go wrong with those.
Best, Chris
Long time, good to "see" you.
Have you considered the 7591EH? That should give spaceistheplace exactly what he wants. Drive requirements aren't that much more than the 6BQ5 either.
Hi spaceistheplace,
For tube types, Electroharmonix from New Sensor are about the best new tubes, and they don't cost a lot either. Skip all the silliness of NOS types and just buy a set of new tubes. I find the Electroharmonix tubes are very consistent from tube to tube, and they also match what a good NOS tube measures. You can't go wrong with those.
Best, Chris
Thanks for the tip! What kind of power and output transformer should I use for the 7591EH? I’d like to use a single center tapped power tx if possible. I was almost ready to start looking at other tube designs as it didn’t seem like there was much consensus on quality or best avenue to pursue with this version of the B.H.
7591 is indeed a good sounding tube and will give you up to 35W. The 35wpc Eico ST70 uses 7591.
Any output transformer of impedance around 6600 will do. However, I have to disagree with Chris, 7591EH is nowhere near the NOS 7951 in sound quality and measurement. My experience very much matches this small sampling comparison. I suspect 7591EH is a 6L6GC in 7591 pin orderings. To my ears, JJ actually sounds closer to the Westinghouse and RCA than EH, just can't be pushed as hard.
If 30wpc is your target, then you do have some new production selections to choose from. At the current price, you can try all 3 brands without breaking the bank.
FWIW, the PT with CT or not is completely irrelevant to the tube type chosen. You can use fullwave bridge (not use CT) or fullwave (requires CT). It is just a matter of power supply design, the tubes don't care how you supply the B+ and careless there is a CT or not.
Marc's board is very well designed and flexible; however, flexibility is also your enemy because you will have to fine tune the configuration if you are going down the path that no one has done. My 35wpc Eico HF87 (converted to fixed bias) sounds excellent with EL34's. You really can't go wrong with the EL34/6CA7.
Any output transformer of impedance around 6600 will do. However, I have to disagree with Chris, 7591EH is nowhere near the NOS 7951 in sound quality and measurement. My experience very much matches this small sampling comparison. I suspect 7591EH is a 6L6GC in 7591 pin orderings. To my ears, JJ actually sounds closer to the Westinghouse and RCA than EH, just can't be pushed as hard.
If 30wpc is your target, then you do have some new production selections to choose from. At the current price, you can try all 3 brands without breaking the bank.
FWIW, the PT with CT or not is completely irrelevant to the tube type chosen. You can use fullwave bridge (not use CT) or fullwave (requires CT). It is just a matter of power supply design, the tubes don't care how you supply the B+ and careless there is a CT or not.
Marc's board is very well designed and flexible; however, flexibility is also your enemy because you will have to fine tune the configuration if you are going down the path that no one has done. My 35wpc Eico HF87 (converted to fixed bias) sounds excellent with EL34's. You really can't go wrong with the EL34/6CA7.
Is the 7591 pinout compatible with the octal sockets on Marc’s boards? It’s between that and the EL34 or KT77 I think from what I understand. The PT / CT comment was in reference to the other posters inquiry about sourcing a transformer specifically for Marc’s boards. It was my understanding that it would split the total available current.
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Hi Fred,
Okay, we'll disagree on that.
As for the 7591EH, you're way off the mark! These are a clone of an actual NOS tube and really are a 7591A. Most Electroharmonix tubes are reverse engineered RCA tubes I was told, and from what I have seen I believe them. The Sovtek 7591XYZ was something else (not a 6L6GC), so maybe you're thinking of that. You have to understand that Sovtek tubes are actually Russian tubes marked with the closest North American numbers for the most part. Maybe not all.
I have had variable experiences with JJ tubes over many, many years compared to very consistent performance and reliability with Electroharmonix and still only 2 problem tubes (1 noisy 12AX7EH and one output tube in the wrong box but otherwise fine). In addition, the real EL-34 is a pentode that tends to burn it's base, and the 6CA7 which is a beam power tube without the habit of eating its base. In every piece of equipment the 6CA7 sounds better than the EL34 does in my experience.
Be careful reading tube "shoot-outs" as most are conducted with guitar amplifiers and the positive traits they want are not shared for reproduction equipment. Still, very few new tubes will last in a Fender Twin, and the 6L6EH is one of them. The comparison you linked to seems to favor the JJ tubes by the author's comments. Unavoidable that someone may have their favorite brand I guess. JJ tubes are priced lower than most, and there is a reason for that ...
The other tube lines that New Sensor has been selling are reverse engineered from that same brand name. What New Sensor brings to the table is higher consistency and better materials simply due to advances in manufacturing. I would gladly sell my NOS tubes for market value and replace them with Electroharmonix tubes.
Hi spaceistheplace,
If you go with a Hammond output transformer you should expect really good response right to the frequency extremes. They aren't afraid to use a lot of iron. I have some prototype transformers made by Hammond for an amp I hope to make in that power range. A 1650G or 1650HA would be perfect, see: Tube Output (10 - 280 Watts) Easy Wire Secondary (1608A-1650A Series) - Hammond Mfg. and click on the model of interest. These will be heavier than most. They have been recently had their designs reviewed and it's possible that Hammond might redesign them in the future as well.
The power transformer will depend on the actual power output you decide on.
-Chris
Okay, we'll disagree on that.
As for the 7591EH, you're way off the mark! These are a clone of an actual NOS tube and really are a 7591A. Most Electroharmonix tubes are reverse engineered RCA tubes I was told, and from what I have seen I believe them. The Sovtek 7591XYZ was something else (not a 6L6GC), so maybe you're thinking of that. You have to understand that Sovtek tubes are actually Russian tubes marked with the closest North American numbers for the most part. Maybe not all.
I have had variable experiences with JJ tubes over many, many years compared to very consistent performance and reliability with Electroharmonix and still only 2 problem tubes (1 noisy 12AX7EH and one output tube in the wrong box but otherwise fine). In addition, the real EL-34 is a pentode that tends to burn it's base, and the 6CA7 which is a beam power tube without the habit of eating its base. In every piece of equipment the 6CA7 sounds better than the EL34 does in my experience.
Be careful reading tube "shoot-outs" as most are conducted with guitar amplifiers and the positive traits they want are not shared for reproduction equipment. Still, very few new tubes will last in a Fender Twin, and the 6L6EH is one of them. The comparison you linked to seems to favor the JJ tubes by the author's comments. Unavoidable that someone may have their favorite brand I guess. JJ tubes are priced lower than most, and there is a reason for that ...
The other tube lines that New Sensor has been selling are reverse engineered from that same brand name. What New Sensor brings to the table is higher consistency and better materials simply due to advances in manufacturing. I would gladly sell my NOS tubes for market value and replace them with Electroharmonix tubes.
Hi spaceistheplace,
If you go with a Hammond output transformer you should expect really good response right to the frequency extremes. They aren't afraid to use a lot of iron. I have some prototype transformers made by Hammond for an amp I hope to make in that power range. A 1650G or 1650HA would be perfect, see: Tube Output (10 - 280 Watts) Easy Wire Secondary (1608A-1650A Series) - Hammond Mfg. and click on the model of interest. These will be heavier than most. They have been recently had their designs reviewed and it's possible that Hammond might redesign them in the future as well.
The power transformer will depend on the actual power output you decide on.
-Chris
Hi Chris,
If it's not secret, I would like to know reason why JJ tubes are priced lower? I might reconsider when I need tubes and maybe JJ can be option to buy or not.
Appreciate your reply on this.
bekim
Hi bekim,
I don't know for sure, but looking at the two offers up some answers. The glass is heavier and plates generally larger on the New Sensor products. I just think that they cost less to produce than the New Sensor line does.
JJ tubes aren't bad or horrible tubes. I've just had a lot more luck with the Electroharmonix tubes. I use them for repairs and also in my own equipment. No way would I use an inferior product for repairs over a cost differential. I see enough tubes to get what I figure is a good feel for a brand of tube.
-Chris
I don't know for sure, but looking at the two offers up some answers. The glass is heavier and plates generally larger on the New Sensor products. I just think that they cost less to produce than the New Sensor line does.
JJ tubes aren't bad or horrible tubes. I've just had a lot more luck with the Electroharmonix tubes. I use them for repairs and also in my own equipment. No way would I use an inferior product for repairs over a cost differential. I see enough tubes to get what I figure is a good feel for a brand of tube.
-Chris
Hi Chris,
No worry for disagree, I just happen to have quite a few original Eico/Westinghouse 7591A as well as RCA (regular and coin base) to compare them from my Eico ST70 and Eico ST40. The EH just don't sound the same as the Westinghouse and the RCA - strong bass but somewhat muddy mid and high. I have no equipment to test and to measure the specs the tubes as David. So, it is simply one's opinion from comparing by ears.
I am in 100% agreement that the EH's are very rugged. I always put them in when I made circuit modification to the Eico's. They are still alive under so many mistakes I have made
Hi spaceistheplace,
7591 and EL34 have different pinouts. If you don't plan to outboard the sockets and rewire , then 7591 is not an option for you at all. In which case, you are very much limited to EL34, 6CA7, 6L6 family, 6550, KT*, 6V6. For around 30wpc, EL34, 6CA7, 6L6, KT66, and KT77 will do. This is still a long list.
As Chris pointed out, the choice of power transformer depends on the tubes, the output power, and the power supply design. Marc's board has full wave bridge capacitor input implemented. Therefore, the CT is really of no use if you plan to use the onboard power supply. In this case, the I D.C = 0.62 x secondary I A.C.
You can estimate the HV secondary as following for one channel:
- Let say I am going to bias the tubes at 45mA, then for 2 tubes, that will be 90mA at idle. I will call this the baseline.
- Since I don't like listening to clipping, I don't expect to drive them hard; however, I may bias them hot at some point; therefore, I think 1.5X of the idle will serve me well. This brings the total to 135mA for one channel.
- To deliver 135mA D.C, the PT will need to provide 135mA / 0.62 = 217mA. For 2 channels, I double this number, so I want a 400mA PT.
Notice that the above is simply an illustration of how I plan my amp. There are many factors that can affect the decision. For example, a good PT coupled with high capacitance caps can deliver the momentary spike demand. Perhaps, a solid 300mA PT will work just fine. In this example, I think the Toroidy meets my requirements. How will I use it? Just connect the HV secondary to the AC inputs of the 2 boards.
No worry for disagree, I just happen to have quite a few original Eico/Westinghouse 7591A as well as RCA (regular and coin base) to compare them from my Eico ST70 and Eico ST40. The EH just don't sound the same as the Westinghouse and the RCA - strong bass but somewhat muddy mid and high. I have no equipment to test and to measure the specs the tubes as David. So, it is simply one's opinion from comparing by ears.
I am in 100% agreement that the EH's are very rugged. I always put them in when I made circuit modification to the Eico's. They are still alive under so many mistakes I have made
Hi spaceistheplace,
7591 and EL34 have different pinouts. If you don't plan to outboard the sockets and rewire , then 7591 is not an option for you at all. In which case, you are very much limited to EL34, 6CA7, 6L6 family, 6550, KT*, 6V6. For around 30wpc, EL34, 6CA7, 6L6, KT66, and KT77 will do. This is still a long list.
As Chris pointed out, the choice of power transformer depends on the tubes, the output power, and the power supply design. Marc's board has full wave bridge capacitor input implemented. Therefore, the CT is really of no use if you plan to use the onboard power supply. In this case, the I D.C = 0.62 x secondary I A.C.
You can estimate the HV secondary as following for one channel:
- Let say I am going to bias the tubes at 45mA, then for 2 tubes, that will be 90mA at idle. I will call this the baseline.
- Since I don't like listening to clipping, I don't expect to drive them hard; however, I may bias them hot at some point; therefore, I think 1.5X of the idle will serve me well. This brings the total to 135mA for one channel.
- To deliver 135mA D.C, the PT will need to provide 135mA / 0.62 = 217mA. For 2 channels, I double this number, so I want a 400mA PT.
Notice that the above is simply an illustration of how I plan my amp. There are many factors that can affect the decision. For example, a good PT coupled with high capacitance caps can deliver the momentary spike demand. Perhaps, a solid 300mA PT will work just fine. In this example, I think the Toroidy meets my requirements. How will I use it? Just connect the HV secondary to the AC inputs of the 2 boards.