I'm fiddling with PP design with two 12K5 spacecharge tubes (those with low anode voltage, max 30V). As the voltages are relatively low and the iron is light I wanted to try an interstage transformer instead of a tube PI.
I've got some cheapo tiny 600:600CT lying around so I used one.
My OT is another cheapo-tiny 1.2KCT to 8ohms.
Everything works but fairly quiet, way more quiet than a SE 12K5 amp. Quiet, but despite the low output there's lots of distortion, sounds a lot like another spacecharge amplifiers with SE out maxed out. I've checked the circuit with a different OT, which is relatively big and has approximately the same ratio, and there was almost no change in the output.
Which leads me to the interstage transformer, which is probably the problem. Unfortunately I don't have any other suitable transformers to try in that spot, so instead of buying some for testing I'm here to ask the wise men. I happen to have no experience with transformer PI's , so sorry if the question is beyond stupid.
I'll post the schematics later, but tell me first, is it possible to use 600:600CT for PI? The output tubes work in grid bias mode, which is the way spacecharge tubes prefer. The transformer's secondary is center-grounded and the signals go through caps to the grids of output tubes, each with 1M ground reference. 12K5's cathodes are grounded.
My intuition tells me transformer's 300 ohms to the ground is way too low to put before a tube with 1M ground reference, so I need at least 2M CT secondary and probably something like 500K primary. Is that correct?
I've got some cheapo tiny 600:600CT lying around so I used one.
My OT is another cheapo-tiny 1.2KCT to 8ohms.
Everything works but fairly quiet, way more quiet than a SE 12K5 amp. Quiet, but despite the low output there's lots of distortion, sounds a lot like another spacecharge amplifiers with SE out maxed out. I've checked the circuit with a different OT, which is relatively big and has approximately the same ratio, and there was almost no change in the output.
Which leads me to the interstage transformer, which is probably the problem. Unfortunately I don't have any other suitable transformers to try in that spot, so instead of buying some for testing I'm here to ask the wise men. I happen to have no experience with transformer PI's , so sorry if the question is beyond stupid.
I'll post the schematics later, but tell me first, is it possible to use 600:600CT for PI? The output tubes work in grid bias mode, which is the way spacecharge tubes prefer. The transformer's secondary is center-grounded and the signals go through caps to the grids of output tubes, each with 1M ground reference. 12K5's cathodes are grounded.
My intuition tells me transformer's 300 ohms to the ground is way too low to put before a tube with 1M ground reference, so I need at least 2M CT secondary and probably something like 500K primary. Is that correct?
2 jazbo: you're right, my mistake. Actual measured resistance of the transformer is a bit lower, around 295Ohms per half. Which still acts like a 300 Ohm reference to the ground.
2stocktrader: that's what I did. I have a couple with multiple primaries and tested the circuit with it. It's quite an overkill for spacecharge purposes, but it worked fine. And the "tiny" one I've used originally puts out approximately the same volume and distortion, without overheating. That's why I'm saying it looks like the problem is in the PI.
2stocktrader: that's what I did. I have a couple with multiple primaries and tested the circuit with it. It's quite an overkill for spacecharge purposes, but it worked fine. And the "tiny" one I've used originally puts out approximately the same volume and distortion, without overheating. That's why I'm saying it looks like the problem is in the PI.
You are still confusing DC resistance with AC impedance... please post the schematic, may be we can spot something...
A "600 ohm to 600 ohm CT" transformer was intended for operation in a low impedance circuit, probably solid state or telephone coupling. Attempting to use it in a high impedance tube circuit will result in poor performance even if the ratios are right because it doesn't have enough primary inductance.
The ratio is the main parameter, but you usually can't get away with using a transformer much beyond a 2 to 1 change from its intended use. A 600 ohm to 600 ohm transformer will probably work OK as a 300 ohm to 300 ohm transformer, but probably not as a 60 ohm to 60 ohm due to excess winding capacitance, nor as a 6000 ohm to 6000 ohm transformer due to lack of inductance (not enough turns of wire). You are asking it for quite a bit more.
There are exceptions of course. A higher quality transformer may be pushed a bit farther because it probably has more inductance and lower capacitance to begin with, and these guidelines don't apply if the frequency range of operation is changed or restricted.
You 600 ohm transformer probably looks like a dead short to the driver tube. You will probably find that the performance improves as the frequency is increased unless it was a telephone(or modem) coupling transformer, they aren't meant to do much above 3 KHz.
The ratio is the main parameter, but you usually can't get away with using a transformer much beyond a 2 to 1 change from its intended use. A 600 ohm to 600 ohm transformer will probably work OK as a 300 ohm to 300 ohm transformer, but probably not as a 60 ohm to 60 ohm due to excess winding capacitance, nor as a 6000 ohm to 6000 ohm transformer due to lack of inductance (not enough turns of wire). You are asking it for quite a bit more.
There are exceptions of course. A higher quality transformer may be pushed a bit farther because it probably has more inductance and lower capacitance to begin with, and these guidelines don't apply if the frequency range of operation is changed or restricted.
You 600 ohm transformer probably looks like a dead short to the driver tube. You will probably find that the performance improves as the frequency is increased unless it was a telephone(or modem) coupling transformer, they aren't meant to do much above 3 KHz.
Thank you for the answers,
I think tubelab more or less nails the problem, it sounds almost like a dead short.
The question then is - what transformer ratio may work in this application? I will not buy some Altec for this amp, it seems right to find something like Xicon -
They have this 42L series, I may try to order something like 2K:10K to see if it works better. Maybe ordering several transformers and testing them may be the best solution - I may order several with different ratios for some $10-15.
http://www.mouser.com/ds/2/449/XC-600127-202003.pdf
This is not going to be a hi-fi amp, that's for sure. Awful distorted guitar amp with cheap parts, this is probably the correct description. By the way, spacecharge tubes sound awesome with guitars, but they're way too quiet. Even too quiet to put a tone stack in the chain. So this is an attempt to get some volume out of spacecharge tubes.
I think tubelab more or less nails the problem, it sounds almost like a dead short.
The question then is - what transformer ratio may work in this application? I will not buy some Altec for this amp, it seems right to find something like Xicon -
An externally hosted image should be here but it was not working when we last tested it.
They have this 42L series, I may try to order something like 2K:10K to see if it works better. Maybe ordering several transformers and testing them may be the best solution - I may order several with different ratios for some $10-15.
http://www.mouser.com/ds/2/449/XC-600127-202003.pdf
This is not going to be a hi-fi amp, that's for sure. Awful distorted guitar amp with cheap parts, this is probably the correct description. By the way, spacecharge tubes sound awesome with guitars, but they're way too quiet. Even too quiet to put a tone stack in the chain. So this is an attempt to get some volume out of spacecharge tubes.
The key words you left out are "to the driver tube", without knowing which tube you are using, it's hard to know which type of IST is suitable for the application. But IST with so called low resistance windings ("a dead short") are used regularly, such as the Lundahl 1660.
You're correct as always, the tubes I'm using are 12AJ6 for preamp and 12K5 for output.
Ha ha, if only that was true... there are a bunch of circuits on this thread including transformer coupled ones for you to check out, and it does seem that the 600:600 transformer isn't really suitable for the job. Also by coincidence, Printer2 recently started a discussion thread on low voltage tube guitar amps that you might be interested in participating.
Thanks for the link.... studied the subject a bit more.
Though it was evident from the beginning that long tail PI will work, I'm still on a mission to find a "midget" transformer for PI. That old scheme shows 7K:15K CT so I will try to find something similar. There will probably be a need for another gain stage, but we'll see.
Though it was evident from the beginning that long tail PI will work, I'm still on a mission to find a "midget" transformer for PI. That old scheme shows 7K:15K CT so I will try to find something similar. There will probably be a need for another gain stage, but we'll see.
Hi,
Looking at your schematic there seems to be something very wrong.
Grid #2 should be biased around -2V which isn't happening here.
You either need a fixed negative voltage supplied to the secondary center of the interstage transformer, AC decoupled to ground.
Or you need a suitable common cathode-resistor to raise the cathodes to around +2V, connecting the xfmr center-tap directly to ground.
In both cases DC-couple the interstage transformer to the grids, no coupling cap's.
Also the para-feed driver is maybe not such a good idea.
A 10nF coupling cap along with the 1Mohm plate resistor form a very high impedance not suitable for driving the low-z primary of the transformer.
With ideal matched circuits how much power will you get from a 12K5 push-pull stage? 50mWatt in the very best cases? That's about what came out of the first pocket-sized transistor-radios and they sounded pretty wimpy. In any case you'll need good, sensitive speakers to enjoy the tiny power from such a construction.
Rgds.
Looking at your schematic there seems to be something very wrong.
Grid #2 should be biased around -2V which isn't happening here.
You either need a fixed negative voltage supplied to the secondary center of the interstage transformer, AC decoupled to ground.
Or you need a suitable common cathode-resistor to raise the cathodes to around +2V, connecting the xfmr center-tap directly to ground.
In both cases DC-couple the interstage transformer to the grids, no coupling cap's.
Also the para-feed driver is maybe not such a good idea.
A 10nF coupling cap along with the 1Mohm plate resistor form a very high impedance not suitable for driving the low-z primary of the transformer.
With ideal matched circuits how much power will you get from a 12K5 push-pull stage? 50mWatt in the very best cases? That's about what came out of the first pocket-sized transistor-radios and they sounded pretty wimpy. In any case you'll need good, sensitive speakers to enjoy the tiny power from such a construction.
Rgds.
spacecharge tubes
Regarding the spacecharge tubes, why I like them:
1. Power supply can be made of some cheesy 12V transformer or simply recycle a laptop or printer power supply. Output transformers may be line 70V, again very low $$$.
2. Spacecharge tubes are still relatively chap.
3. No deadly voltages.
4. Guitar-vise, distortion and saturation may be achieved at low volumes. It is good for recording and practicing at home.
5. They sound good.
Regarding the spacecharge tubes, why I like them:
1. Power supply can be made of some cheesy 12V transformer or simply recycle a laptop or printer power supply. Output transformers may be line 70V, again very low $$$.
2. Spacecharge tubes are still relatively chap.
3. No deadly voltages.
4. Guitar-vise, distortion and saturation may be achieved at low volumes. It is good for recording and practicing at home.
5. They sound good.
Do you have soundclips that you can share? All the ones I have heard so far sounded pretty "thin"...
demo
I really need to make some demo to show how it sounds. I will one day.
Still,
Here's a song with a mix of two different amps, two takes through a single-ended spacecharge amp with 12AU7 output tube and two through a 30W AC-30 like amp with Marshall BluesBreaker pedal in front of it. Spacecharger is cleaner and it brings clarity to the mix. The guitar is '83 Gibson Challenger, upgraded with $10 noname Chinese pickup.
Just in case someone thinks it's thin, you know...
https://soundcloud.com/sergey-engel/altalena-generation-comes-1
I really need to make some demo to show how it sounds. I will one day.
Still,
Here's a song with a mix of two different amps, two takes through a single-ended spacecharge amp with 12AU7 output tube and two through a 30W AC-30 like amp with Marshall BluesBreaker pedal in front of it. Spacecharger is cleaner and it brings clarity to the mix. The guitar is '83 Gibson Challenger, upgraded with $10 noname Chinese pickup.
Just in case someone thinks it's thin, you know...
https://soundcloud.com/sergey-engel/altalena-generation-comes-1
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Jazbo8, i have some clips of my otl's whith 12k5 for you replay.
Very good tone and solutións, and pro for audiphone solutións.
Amplificadores de Valvulas OTL sin Transformador de Salida, Muestras de Sonido.
Very good tone and solutións, and pro for audiphone solutións.
Amplificadores de Valvulas OTL sin Transformador de Salida, Muestras de Sonido.
Thanks !
The speaker is one vintage 600 ohms, and it help to good and spaciate sound whit volumetric type of proyection.
Is know the limited power, but very good amp for recording and home/study.
For more power y have one tipe philips biamp whit 2 x el86 an other identical whit 2 EL34, and 5 watts aprox.
The speaker is one vintage 600 ohms, and it help to good and spaciate sound whit volumetric type of proyection.
Is know the limited power, but very good amp for recording and home/study.
For more power y have one tipe philips biamp whit 2 x el86 an other identical whit 2 EL34, and 5 watts aprox.
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