I'm a little confused about how to find the optimum current to run these tubes. The voltages we're running them at is off the charts (350 - 400v.. chart only goes to ~340). Is the idea to keep the same ratio, as if the line were on the chart?
My power transformer is supposed to be 350-0-350, but I'm getting over 400v at the tubes. I currently have the bias set to about 58ma. I'm not really using my amp right now because of the high voltage, but I plan to fix it this weekend.
I'm going to try getting my voltage down to 350. At that point I guess I'll try 65ma, like Mr TubeLab.
All of this is way off the charts, and I'm only doing it based on the couple of posts specifying this sort of info here on the forums.
So, just to double check... does that sound about right, or should I shoot for some other values while I'm working on the amp.
This is in a 300b s.e. being converted to 307a
thanks a lot!
My power transformer is supposed to be 350-0-350, but I'm getting over 400v at the tubes. I currently have the bias set to about 58ma. I'm not really using my amp right now because of the high voltage, but I plan to fix it this weekend.
I'm going to try getting my voltage down to 350. At that point I guess I'll try 65ma, like Mr TubeLab.
All of this is way off the charts, and I'm only doing it based on the couple of posts specifying this sort of info here on the forums.
So, just to double check... does that sound about right, or should I shoot for some other values while I'm working on the amp.
This is in a 300b s.e. being converted to 307a
thanks a lot!
Maybe someone has a better take on this, but to my eye, triode connected these look like neither 2A3's nor 300B's so much as 6L6's. Triode connected at least, Rp is slightly lower than a 6L6 (1K5 v 1K7) as is plate dissipation (15W v 19W), though the indication seems to be that pushing these a little is fine.
Conservatively, I think my starting point would be 310V on the plate, -30V on the grid, and ~45mA. Even pushing it to higher dissipation, that -30V on the grid seems about right to me.
Conservatively, I think my starting point would be 310V on the plate, -30V on the grid, and ~45mA. Even pushing it to higher dissipation, that -30V on the grid seems about right to me.
dsavitsk, you're right, the curves are pretty close to a triode-connected 6L6...
I've been running B+ of 400V and a shared cathode bias resistor of 500 ohms in push-pull... somewhere around 40-45V on the cathodes, I think, so between 40 and 45mA and ~360V across the tubes. So far they seem happy there.
Pete
I've been running B+ of 400V and a shared cathode bias resistor of 500 ohms in push-pull... somewhere around 40-45V on the cathodes, I think, so between 40 and 45mA and ~360V across the tubes. So far they seem happy there.
Pete
dsavitsk said:
I haven't tried anything higher than 8k in P-P (I don't have any 10k's)... but less than 8K P-P probably isn't as good. In SE I found 5k is better than 3.5k, so I would guess that 10k P-P would work well.
The most power I've been able to squeeze out of a P-P pair is around 12W @ 5% THD, so I think you'd be OK with a 15W transformer.
Pete
Small 307A amplifier with 5K output trannies from old radio receiver:
307a Single End Amplifier
Dragan
307a Single End Amplifier
Dragan
They are pentodes with a 5 pin base. I made a crude adapter wired them into my TSE, set the bias to about 60 ma and let it rip. That was more than 2 years ago and it is still running. It doesn't see a lot of use because it was never built into a proper cabinet, but it sounds good. I am not running them at 400 volts though. My TSE has 300-0-300 power transformer. It is a Hammond so it is really a 325-0-325 and the B+ is about 350 to 360 volts depending on the whip of the local power company at any given moment.
The WE spec seems rather conservative at 15 watts plate dissipation and I ran them at 30 watts for a few days, but they sound about the same at 20 to 25 watts.
On a slightly different note another poster in this thread that might be named Pete made this big red board for use with sweep tubes. I got tired of playing with sweep tubes so I stuck some 307A's in the red board. It works, makes about 30 WPC without melting anything. Sounds good too. I haven't had time to play with it in a few months, but I will get back to it.
Thank you, that is helpful. I am building my 300b TSE, I bought your boards, but may end up p-p wiring, but at least I feel I have paid something for your intellectual property. Thank you. In any case, by going p-p and mounting the tubes off the board, I could swap pin bases more easily and try the 307A's.
Although off topic, might I ask some advice here? My power transformer is somewhat overpowered, even for the 300b's and i would rather not buy another. What are your recommendations for reducing the B+? I can reduce C4, add another RC filter ( I am using CLC), add transformer resistors in line with B+, buck transformer (although I don't think I have a coil for this) or..? Thanks again.
It depends on just how overpowered your transformer is. The simplest method is to add a second choke between the 5AR4 and the first cap turning the CLC onto a LCLC or even a LCRC. The usual risk here is that the B+ can rise to excessive levels if the B+ comes up before the output tubes are conducting. This shouldn't be an issue with DHT's (quick starting) and a 5AR4 (slow). Don't try this with a solid state rectifier or slow starting output tubes.
Sometimes experimenting is needed to verify the output voltage and to see if your choke will buzz when operated as an input choke. A choke input filter needs a load to operate or the output voltage will be too high. find something that draws about the same current as the amp will when operating.
Sometimes experimenting is needed to verify the output voltage and to see if your choke will buzz when operated as an input choke. A choke input filter needs a load to operate or the output voltage will be too high. find something that draws about the same current as the amp will when operating.
Ok , not so smart a question,but what would I use for that?
150mA,400V,maybe a 60W and 2,600R?
Thanks, I guess I was trying to imagine something more elegant. Ah, well...
I have read some articles about power supplies by DHTRob and he advocates adding some resistance in series with the anode of the rectifier tube to reduce current spikes and ringing in the PS. For some reason I can't cut and paste his statement regarding this. Doing this and modelling in PSUDII with only a modest amount of added resistance seems to allow some reduction in voltage as well, certainly easy and adequate for my needs.
Do you see any significant disadvantage in this, as long as reasonable resistances are considered like 100R or so?
Another option would be to add a shunt regulator. I have a Welborne labs PS-3, if I can figure it out, or I could build the one by Salas. More complex, but with added regulation. Doesn't seem like much additional regulation is needed as the PS has very little ripple as is, but it would allow me to have some control over voltage.
I save "elegant" for the finished amp. A dummy load is only needed for testing the power supply and then it all goes back in the junk box. I won't post some of the unsafe contraptions I have used to test big power supplies since some involve water and electricity!
No, it is a valid way to drop a few volts and it does reduce the ripple current in the power supply which lowers the "crud" level that can find its way into other circuits. it also makes life easier on the rectifier tube.
In a Class A amp like this you can add quite a bit of resistance. In a class AB push pull amp too much resistance will cause the power supply voltage to drop under load resulting in distortion and other effects that may be desired in a guitar amp.
I just discovered this tube...and this thread.
The VT-225 was apparently only used in a particular army field radio, the BC-654A. I obtained one of these radios last summer from an estate sale and today happened to pull the guts out of the cabinet to see what I had. I saw three of those tubes in there and at first thought they were a TV sweep tube I have in my collection since the plate structure and bottle shape was at first glance nearly identical to a 6CB5. Then I noticed they had DH cathodes and basically "got excited'.
Well the label on the glass said RK-75. I'd never ever heard of this tube and after pulling one out saw the VT-225 label on the base. My set of three are the according to one website 'apparently rare' 3-rib plate Raytheon version. I had absolutely never heard of anyone using this tube in a SET amp before but it sure looked like a potential candidate. A quick google search led me to this thread.
OK, so I think I'll try to find time to play with these. If I can get a few more as spares I'll perhaps make an amplifier. 5-6 watts would be plenty for my basement system on my high efficiency Ultor speaker prototypes. On these speakers I'm currently running 4-125A's in A1 SET (G2 strapped to plate) on 250 volts and 45 mA (~11 watts input), clip leaded and sitting on the carpet, no breadboard. Been running this like that for the past four years. Heater consumption greatly exceeds output power.
DC heat was a must as on AC the hum was terrible. These sound impressive here BTW. I figured on bumping the B+ up to around 600 volts for more output and putting them in a chassis someday. I worried about the lack of gettering caused by such low plate power but after many, many hours of such idling they don't seem to have changed at all.
Come to think about it, I should just clip lead these VT-225's into that setup and drop the filament supply to 5 volts with a mains variac.. I could be listening to them tonight. OK, where did I put my variac?
The VT-225 was apparently only used in a particular army field radio, the BC-654A. I obtained one of these radios last summer from an estate sale and today happened to pull the guts out of the cabinet to see what I had. I saw three of those tubes in there and at first thought they were a TV sweep tube I have in my collection since the plate structure and bottle shape was at first glance nearly identical to a 6CB5. Then I noticed they had DH cathodes and basically "got excited'.
Well the label on the glass said RK-75. I'd never ever heard of this tube and after pulling one out saw the VT-225 label on the base. My set of three are the according to one website 'apparently rare' 3-rib plate Raytheon version. I had absolutely never heard of anyone using this tube in a SET amp before but it sure looked like a potential candidate. A quick google search led me to this thread.
OK, so I think I'll try to find time to play with these. If I can get a few more as spares I'll perhaps make an amplifier. 5-6 watts would be plenty for my basement system on my high efficiency Ultor speaker prototypes. On these speakers I'm currently running 4-125A's in A1 SET (G2 strapped to plate) on 250 volts and 45 mA (~11 watts input), clip leaded and sitting on the carpet, no breadboard. Been running this like that for the past four years. Heater consumption greatly exceeds output power.
DC heat was a must as on AC the hum was terrible. These sound impressive here BTW. I figured on bumping the B+ up to around 600 volts for more output and putting them in a chassis someday. I worried about the lack of gettering caused by such low plate power but after many, many hours of such idling they don't seem to have changed at all.Come to think about it, I should just clip lead these VT-225's into that setup and drop the filament supply to 5 volts with a mains variac.. I could be listening to them tonight. OK, where did I put my variac?
The VT-225 was apparently only used in a particular army field radio, the BC-654A. I obtained one of these radios last summer from an estate sale and today happened to pull the guts out of the cabinet to see what I had. I saw three of those tubes in there and at first thought they were a TV sweep tube I have in my collection since the plate structure and bottle shape was at first glance nearly identical to a 6CB5. Then I noticed they had DH cathodes and basically "got excited'.
Well the label on the glass said RK-75. I'd never ever heard of this tube and after pulling one out saw the VT-225 label on the base. My set of three are the according to one website 'apparently rare' 3-rib plate Raytheon version. I had absolutely never heard of anyone using this tube in a SET amp before but it sure looked like a potential candidate. A quick google search led me to this thread.
OK, so I think I'll try to find time to play with these. If I can get a few more as spares I'll perhaps make an amplifier. 5-6 watts would be plenty for my basement system on my high efficiency Ultor speaker prototypes. On these speakers I'm currently running 4-125A's in A1 SET (G2 strapped to plate) on 250 volts and 45 mA (~11 watts input), clip leaded and sitting on the carpet, no breadboard. Been running this like that for the past four years. Heater consumption greatly exceeds output power.
Come to think about it, I should just clip lead these VT-225's into that setup and drop the filament supply to 5 volts with a mains variac.. I could be listening to them tonight. OK, where did I put my variac?
OK, got a round tuit today. I have the RK-75's operating in my carpetboard amp in the basement now and they are running as follows.
Triode connected with g2 and g3 tied directly to the plate.
Vp= 237 VDC
Ip +Ig3+ Ig2= 55 mA.
Grid bias = -21 VDC.
Dissipation total 13 watts.
How do they sound on my near 100 dB efficicient Ultor speakers?
WoW! Compared to the good sounding 4-125A's, which I was running SET (g2 tied to plate), before in the same carpetboard setup these have just as good impressive bass authority but a layer has been peeled off the highs. I've just heard stuff in my recordings way up high in the tinkle department that I never heard before.
Tubes are showing a slight bit of blue around the glass and are staying quite cool at this power level. At 5.5 VDC on the cathodes they are dark emitters like the 300B. I am not running local FB around this output stage.
I should plop in a couple of grid stoppers and then just listen to these for a while.
Must find some spares of these now so I can safely build an amp into a chassis.
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