Hey all,
I'm interested in building a small amp for desktop and headphone use, using the SSE design as a starting point.
I'm thinking currently of 6V6GT/A with the operating point of:
B+ -- 275V
Vp -- 250V
Kr -- 390R
Opt -- 5K
* 1) For plate voltage of 250V with 275V B+, does that occur simply from the voltage drop across the output transformer or is anything else needed?
I've been looking around here for which mods on the SSE are needed to support lower voltages (around 275V B+) and found:
-- Jumper instead of 10K resistor to CCS
-- If using 12AT7, increase current set resistor to 470R
-- Alternatively, use 12BH7A. Although I'm not sure if any further changes are needed except swapping the tube.
* 2) Any other frontend changes needed?
Also, I will attempt to "roll my own" PCB for the SSE minus the power supply. This is to better fit the design I have in mind for a smaller chassis.
It is in early stages, I like the symmetry but not too happy about "weaving" the pentode plate trace next to the signal. I'll probably have to rotate that tube and figure something out.
* 3) Has anyone tried to use a 12AX7 ? I've got alot more of those than 12AT7, and since I'm making the PCB I could incorporate any changes needed to support that.
Thanks in advance.
I'm interested in building a small amp for desktop and headphone use, using the SSE design as a starting point.
I'm thinking currently of 6V6GT/A with the operating point of:
B+ -- 275V
Vp -- 250V
Kr -- 390R
Opt -- 5K
* 1) For plate voltage of 250V with 275V B+, does that occur simply from the voltage drop across the output transformer or is anything else needed?
I've been looking around here for which mods on the SSE are needed to support lower voltages (around 275V B+) and found:
-- Jumper instead of 10K resistor to CCS
-- If using 12AT7, increase current set resistor to 470R
-- Alternatively, use 12BH7A. Although I'm not sure if any further changes are needed except swapping the tube.
* 2) Any other frontend changes needed?
Also, I will attempt to "roll my own" PCB for the SSE minus the power supply. This is to better fit the design I have in mind for a smaller chassis.
It is in early stages, I like the symmetry but not too happy about "weaving" the pentode plate trace next to the signal. I'll probably have to rotate that tube and figure something out.
* 3) Has anyone tried to use a 12AX7 ? I've got alot more of those than 12AT7, and since I'm making the PCB I could incorporate any changes needed to support that.
Thanks in advance.
Attachments
You should not drop more than a few volts through the OPT.
In a cathode biased stage, the voltage drop occurs across the cathode resistor. Measure the voltage at the top end of the cathode resistor, and subtract that from the voltage on the plate, the difference is the voltage across the tube. The voltage drop across the cathode resistor also establishes the negative bias voltage on the control grid.
Going from memory, the grid bias voltage for 6V6 at 250 volts is about - 12 volts, or, in other words, you want to pick Rk to drop about 12 volts across the tube.
So, with 275 volts B+, you'll be seeing 262 ish across the tube.
George has said that some 12AX7 are capable of performing spectacularly when CCS loaded, and some are dogs. You would need to be prepared to experiment, I think.
In a cathode biased stage, the voltage drop occurs across the cathode resistor. Measure the voltage at the top end of the cathode resistor, and subtract that from the voltage on the plate, the difference is the voltage across the tube. The voltage drop across the cathode resistor also establishes the negative bias voltage on the control grid.
Going from memory, the grid bias voltage for 6V6 at 250 volts is about - 12 volts, or, in other words, you want to pick Rk to drop about 12 volts across the tube.
So, with 275 volts B+, you'll be seeing 262 ish across the tube.
George has said that some 12AX7 are capable of performing spectacularly when CCS loaded, and some are dogs. You would need to be prepared to experiment, I think.
Ah, thanks for the explanation regarding the voltage drop.
I'm using the data from the chart on the last page of this experiment:
http://www.r-type.org/pdfs/6v6-se-ul.pdf
Likely use B+ of 275V or 300V with the corresponding cathode resistors there.
I've got a nice selection of NOS 12AX7's to experiment with -- Mullard, Amperex, Telefunken, RCA, Raytheon etc..
For this, do I simply need to modify the current set resistor to get about 1.2mA ?
One other question regarding the frontend CCS .. how is the voltage on the plate determined?
I'm using the data from the chart on the last page of this experiment:
http://www.r-type.org/pdfs/6v6-se-ul.pdf
Likely use B+ of 275V or 300V with the corresponding cathode resistors there.
I've got a nice selection of NOS 12AX7's to experiment with -- Mullard, Amperex, Telefunken, RCA, Raytheon etc..
For this, do I simply need to modify the current set resistor to get about 1.2mA ?
One other question regarding the frontend CCS .. how is the voltage on the plate determined?
Plate voltage on the input tube is determined by the cathode resistor. Most 12AT7's do fine with a fixed value, but the 5842's used in the TSE need a pot since they vary a lot. If you plan to roll a lot of different tubes through your amp at least start with a pot in the cathode circuit.
Some 12AX7's can make very low distortion values when CCS loaded. Their lack of drive capability makes this rather hard to obtain in a simple circuit due to the load imparted by the output stage. My low THD measurements were made in a buffered design like the TSE.
I had a triode wired 6V6GT based SSE and it was a nice sounding amp. I ran a B+ of 325 volts with about 35 to 40 mA of current. My only changes to the design was replacing the 10K resistor in series with the CCS with a wire. I tried two different OPT's, a 3K Transcendar custom designed for 300B's wired for 6K, and the $29 Edcor XSE15-8-5K. I actually preferred the Edcor. At the 2.5 watt power level obtained from a 6V6 the Edcor could do bass far lower than my speakers could, and the top end went to nearly 40 KHz. Can't do that with a 5 pound OPT.
Virtually all 6V6's can eat well over the 315 volt plate voltage rating. I have seen guitar amps run them at 400 volts, so I ran my pair of vintage grey glass RCA's at 320 volts putting 300 volts across the tube.
Some 12AX7's can make very low distortion values when CCS loaded. Their lack of drive capability makes this rather hard to obtain in a simple circuit due to the load imparted by the output stage. My low THD measurements were made in a buffered design like the TSE.
I had a triode wired 6V6GT based SSE and it was a nice sounding amp. I ran a B+ of 325 volts with about 35 to 40 mA of current. My only changes to the design was replacing the 10K resistor in series with the CCS with a wire. I tried two different OPT's, a 3K Transcendar custom designed for 300B's wired for 6K, and the $29 Edcor XSE15-8-5K. I actually preferred the Edcor. At the 2.5 watt power level obtained from a 6V6 the Edcor could do bass far lower than my speakers could, and the top end went to nearly 40 KHz. Can't do that with a 5 pound OPT.
Virtually all 6V6's can eat well over the 315 volt plate voltage rating. I have seen guitar amps run them at 400 volts, so I ran my pair of vintage grey glass RCA's at 320 volts putting 300 volts across the tube.
I've seen that paper before, but I've only skimmed it - I've not read it in depth.
The JJ / Tesla 6V6S which is the subject of the article is not the same as NOS 6V6. In addition to the spiral filament, it also carries a 500 volt plate / 450 volt screen grid rating, and is rated at 450 volts on the plate and screen in triode. The curves are also different; it biases different as well, so the results in that paper may or may not translate to regular 6V6. One would need to run a GE or Sylvania through the same battery of tests and conditions.
I've put the data for the JJ and the generic GE 6V6 to this post.
I've had and used some JJ 6V6S in the past, but I can't find much in my notes about it. All I can find is that I ran it at plate + 300; cathode voltage +15.9; Rk 300 ohms; so 53 milliamps and 15 watts. This was in triode mode; the JJ is rated at 10 watts in triode. A GE 6V6 will set at about +18 volts on the cathode under these same conditions. After about 60 hours of use at the 50% overload, I tested the 6V6S on my TV-7 and they had about 85% of the transconductance of a NOS GE. Whether this was a weakening of the tube from the extreme overload, or about the norm for the 6V6S cannot be gleaned from my old and poorly written notes.
The 6V6S is probably the best of the new production 6V6 by a long shot. I've gotten poor results from the EH 6V6 over several sets of tubes from reputable vendors. I would avoid them; even under the mild operating conditions in the child resistant SSE, they get crackly in a short period of time. ( the child resistant SSE has actually been running GE 6AQ5 for the last four or five years without issue ). I've used the Shuguang 6V6GT ( the ones Stan at ESRC used to sell ) reliably at 300 volts. I don't think I have used any other new 6V6.
I used to have a large inventory of "new" production tubes, but about a decade ago, I gave almost all of them to my nephew, the guitar player, and I don't have many exemplars left. I have some JJ 6L6GC left, I think, but no 6V6S.
I agree with Tubelab that all of the old 6V6, with the possible exception of the 5992, are good for much more than their ratings suggest. As the JJ is also.
The JJ / Tesla 6V6S which is the subject of the article is not the same as NOS 6V6. In addition to the spiral filament, it also carries a 500 volt plate / 450 volt screen grid rating, and is rated at 450 volts on the plate and screen in triode. The curves are also different; it biases different as well, so the results in that paper may or may not translate to regular 6V6. One would need to run a GE or Sylvania through the same battery of tests and conditions.
I've put the data for the JJ and the generic GE 6V6 to this post.
I've had and used some JJ 6V6S in the past, but I can't find much in my notes about it. All I can find is that I ran it at plate + 300; cathode voltage +15.9; Rk 300 ohms; so 53 milliamps and 15 watts. This was in triode mode; the JJ is rated at 10 watts in triode. A GE 6V6 will set at about +18 volts on the cathode under these same conditions. After about 60 hours of use at the 50% overload, I tested the 6V6S on my TV-7 and they had about 85% of the transconductance of a NOS GE. Whether this was a weakening of the tube from the extreme overload, or about the norm for the 6V6S cannot be gleaned from my old and poorly written notes.
The 6V6S is probably the best of the new production 6V6 by a long shot. I've gotten poor results from the EH 6V6 over several sets of tubes from reputable vendors. I would avoid them; even under the mild operating conditions in the child resistant SSE, they get crackly in a short period of time. ( the child resistant SSE has actually been running GE 6AQ5 for the last four or five years without issue ). I've used the Shuguang 6V6GT ( the ones Stan at ESRC used to sell ) reliably at 300 volts. I don't think I have used any other new 6V6.
I used to have a large inventory of "new" production tubes, but about a decade ago, I gave almost all of them to my nephew, the guitar player, and I don't have many exemplars left. I have some JJ 6L6GC left, I think, but no 6V6S.
I agree with Tubelab that all of the old 6V6, with the possible exception of the 5992, are good for much more than their ratings suggest. As the JJ is also.
I thought you were thinking octal 6V6. I would like my heaters supplied by twisted wires, rather than traced on the PCB. That also leaves the flexibility of using tubes with dissimilar voltages - think 5V6 or 12V6. A jumper to pin 9 for the center tube would further help - think 6DT8 rolling 🙂
I was thinking 6V6 originally but I didn't really like the routing around the octal tube.
I had another thought that I'll try out.... perhaps I'll just put a fairly large hole where the output tubes are, and that way they can be mounted on the chassis and 9-pin or octal could be used with the same driver set up.
I had another thought that I'll try out.... perhaps I'll just put a fairly large hole where the output tubes are, and that way they can be mounted on the chassis and 9-pin or octal could be used with the same driver set up.