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Guitar Amps without Isolation Transformer
Checked circuits in Aspen Pitman's 'The Tube Amp Book' 4th Ed.
Found 5 as follows- Harmony 400A, with 12AU6, 50C5 & 35W4 Kay 500, with 12AU6, 50L6 & 35Z5. Also 12AT6 for Tremolo Kay 703-C, with 18GD6, 60FX5, 36AM3 Supra S6611, with 12AU6, 50C5 & Diode Rectifier MI 401, with 20EZ7, 45B5, 36AM3 Oddly, nothing with halfwave voltage doublers which could have doubled OP power at low cost. Most use a simple paralled RC pair to isolate the input stage from the rectifier circuit negative lead. Component values set to limit leakage current to about 2 mA even when the power plug was inserted wrong way around. The Harmony 400A uses a 1:1 transformer in series with the heater 150 mA cct to apply the resulting 150 mA on the other side to the 12AU6 heater. That would help the hum problem. The MI-401 uses an input transformer to isolate the operator. Cheers, John |
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#2
Posted to rec.audio.tubes
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Guitar Amps without Isolation Transformer
"John L Stewart" Oddly, nothing with halfwave voltage doublers which could have doubled OP power at low cost. ** If a maker did that, the DC supply is +/- 170 volts with neutral/ground at the centre point. So, the signal common is now -170 volts relative to ground. Which way around is the correct ( ie safe) one for the AC plug ?? Most use a simple paralled RC pair to isolate the input stage from the rectifier circuit negative lead. Component values set to limit leakage current to about 2 mA even when the power plug was inserted wrong way around. ** Long as that .047uF cap does not fail. BTW: These amps become quite lethal if used in a 240VAC country with a step-down tranny that does not provide isolation - ie an auto-transformer. The voltage across that poor little cap can easily become 240VAC and it will fail very soon. Auto-transformers suitable for such small guitar amps ARE on sale at retail stores here in Australia as well as over the internet. http://www.jaycar.com.au/productView...SUBCAT ID=449 A similar problem applies to older US made guitar amps with two pin plugs that employ isolation transformers but include a " death cap" wired from the AC supply to chassis. Private importation of such amps into Australia is now quite common because of eBay. ..... Phil |
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#3
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Screen supply thru a dropping R helps stabilize the operating point. All works well, I've done it several times. Cheers, John |
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#4
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A full-wave doubler is a nogo for this application. That really needs an isolation transformer. BTW, I use a Hammond One KW auto transformer here to cut firewood in the bush. I ran 110/220 underground all the way to the back of the property. That way I can run a large 110 volt chain saw with no problem. Cheers, John |
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#5
Posted to rec.audio.tubes
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Guitar Amps without Isolation Transformer
In article ,
John L Stewart wrote: Checked circuits in Aspen Pitman's 'The Tube Amp Book' 4th Ed. Found 5 as follows- Harmony 400A, with 12AU6, 50C5 & 35W4 Kay 500, with 12AU6, 50L6 & 35Z5. Also 12AT6 for Tremolo Kay 703-C, with 18GD6, 60FX5, 36AM3 Supra S6611, with 12AU6, 50C5 & Diode Rectifier MI 401, with 20EZ7, 45B5, 36AM3 Oddly, nothing with halfwave voltage doublers which could have doubled OP power at low cost. I would be curious to see a sketch of how you would propose incorporating a voltage doubler in an amp like these at "low cost"? There are three or four design issues that appear difficult to resolve at "low cost". I would think a push-pull output stage would be a better way to increase the power at low cost. Most use a simple paralled RC pair to isolate the input stage from the rectifier circuit negative lead. Component values set to limit leakage current to about 2 mA even when the power plug was inserted wrong way around. I didn't understand what you meant by isolating the input stage this way until I looked up the schematic of the Harmony 400A on the Web, do the other amps you mention use a similar "isolation" circuit technique between the input and output stags? The Harmony 400A uses a 1:1 transformer in series with the heater 150 mA cct to apply the resulting 150 mA on the other side to the 12AU6 heater. That would help the hum problem. I don't understand how the 1:1 transformer "would help the hum problem" to any significant extent? I would think that the major hum problem with this circuit is that any AC voltage that appears across the "paralled RC pair" isolation network will be applied directly to the grid of the output tube. The MI-401 uses an input transformer to isolate the operator. This seems like a far better way to provide isolation, albeit at a significant cost. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#6
Posted to rec.audio.tubes
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Guitar Amps without Isolation Transformer
On 05/14/11 18:20, flipper so wittily quipped:
On Sat, 14 May 2011 20:07:25 +0000, John L Stewart wrote: Checked circuits in Aspen Pitman's 'The Tube Amp Book' 4th Ed. Found 5 as follows- Harmony 400A, with 12AU6, 50C5& 35W4 Kay 500, with 12AU6, 50L6& 35Z5. Also 12AT6 for Tremolo Kay 703-C, with 18GD6, 60FX5, 36AM3 Supra S6611, with 12AU6, 50C5& Diode Rectifier MI 401, with 20EZ7, 45B5, 36AM3 Oddly, nothing with halfwave voltage doublers which could have doubled OP power at low cost. Most of the series string OP tubes were 110V rated anyway, originating for AA5 radios, so a voltage doubler would have been seriously over voltage. another problem might be high heater:cathode volts causing hum in the pre-amp circuits, etc. . using high value cathode bypass capacitors would have driven the cost up and without a PT a separate bias voltage could've been more difficult. Still someone may have 'found a way' so it could be interesting to see if anyone resolved this. |
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#7
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There are a couple of double diode rectifiers that would be OK hooked up as half-wave voltage doublers, still in that 35Z5 octal socket. They would be either 50Y6GT or 117Z7GT. Both have 350 volt heater-cathode rating. So with one extra electrolytic cap something more than 200 volts DC becomes available from a 110 VAC supply. The 50Y6GT satisfies the 150 mA heater string & if the 50L6GT is left as before the other 12 or more volts can run a 12AX7 or 12AU6, just like in the original design. And no need for a heater dropping resistor. A 2 watt dropping resister & another electrolytic supply screen volts to the 50L6GT & if you like the plate supply for the AX7 or AU7. But they are needed in the original circuit as well. So looks like would be possible to get about a 3 db increase in power with only the extra electrolytic in the doubler circuit. We will also need a bit more iron in the OPT. Using the 117Z6GT rectifier is not as elegant a solution but would work anyway. But you now need a heater dropping resistor. PP OP would no doubt deliver more power but brings with it other issues. These would include a CT OPT, phase inverter, extra octal socket, etc. Then there is the problem of enough heater volts. Two 50L6GT’s don’t leave anything for a tube rectifier. That could be run on a separate heater circuit. Or it would be possible to use 25L6’s instead on a 300 mA heater string. That translates to about double the heat, another problem. And you would need a rectifier capable of a bit more than 100 mA. The 1:1 transformer secondary running the 12AU6 heater is tied directly to the –ve supply lead, so very little modulation of its cathode as otherwise may occur. All academic now since this kind of technology would not get a pass from UL, CSA or anybody else! Cheers, John |
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#8
Posted to rec.audio.tubes
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Guitar Amps without Isolation Transformer
On 05/15/11 14:42, John L Stewart so wittily quipped:
The 50Y6GT satisfies the 150 mA heater string& if the 50L6GT is left as before the other 12 or more volts can run a 12AX7 or 12AU6, just like in the original design. And no need for a heater dropping resistor. sounds like the right approach. All academic now since this kind of technology would not get a pass from UL, CSA or anybody else! Academic, yeah. Still it might get approval if you can double insulate it. You would have to make it ridiculously impossible for fingers to come in contact with the chassis, or make the chassis 'earth ground' and isolate the amp itself from the chassis ground. It's possible a power transformer would soon become a cheaper alternative. |
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#9
Posted to rec.audio.tubes
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Guitar Amps without Isolation Transformer
In article ,
John L Stewart wrote: John Byrns;931528 Wrote: In article , John L Stewart wrote: - Checked circuits in Aspen Pitman's 'The Tube Amp Book' 4th Ed. Found 5 as follows- Harmony 400A, with 12AU6, 50C5 & 35W4 Kay 500, with 12AU6, 50L6 & 35Z5. Also 12AT6 for Tremolo Kay 703-C, with 18GD6, 60FX5, 36AM3 Supra S6611, with 12AU6, 50C5 & Diode Rectifier MI 401, with 20EZ7, 45B5, 36AM3 Oddly, nothing with halfwave voltage doublers which could have doubled OP power at low cost.- I would be curious to see a sketch of how you would propose incorporating a voltage doubler in an amp like these at "low cost"? There are three or four design issues that appear difficult to resolve at "low cost". I would think a push-pull output stage would be a better way to increase the power at low cost. - Most use a simple paralled RC pair to isolate the input stage from the rectifier circuit negative lead. Component values set to limit leakage current to about 2 mA even when the power plug was inserted wrong way around.- I didn't understand what you meant by isolating the input stage this way until I looked up the schematic of the Harmony 400A on the Web, do the other amps you mention use a similar "isolation" circuit technique between the input and output stags? - The Harmony 400A uses a 1:1 transformer in series with the heater 150 mA cct to apply the resulting 150 mA on the other side to the 12AU6 heater. That would help the hum problem.- I don't understand how the 1:1 transformer "would help the hum problem" to any significant extent? I would think that the major hum problem with this circuit is that any AC voltage that appears across the "paralled RC pair" isolation network will be applied directly to the grid of the output tube. - The MI-401 uses an input transformer to isolate the operator.- This seems like a far better way to provide isolation, albeit at a significant cost. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ Hi John- That is a very good question. First of all when I look at how a design could be improved I consider what devices were available to the designer at the time. Then, how could those be assembled in the package & hardware that were used. Hi John, I'm glad that you stuck to what was available to the designer at the time, whatever precise time that may have been. There are a couple of double diode rectifiers that would be OK hooked up as half-wave voltage doublers, still in that 35Z5 octal socket. They would be either 50Y6GT or 117Z7GT. Both have 350 volt heater-cathode rating. So with one extra electrolytic cap something more than 200 volts DC becomes available from a 110 VAC supply. The 50Y6GT satisfies the 150 mA heater string & if the 50L6GT is left as before the other 12 or more volts can run a 12AX7 or 12AU6, just like in the original design. And no need for a heater dropping resistor. A 2 watt dropping resister & another electrolytic supply screen volts to the 50L6GT & if you like the plate supply for the AX7 or AU7. But they are needed in the original circuit as well. An excellent proposal, the 50Y6GT is an unusual tube that wasn't very common, at least in my experience. For whatever reason voltage doublers seem to have been mostly used in equipment that used a field coil speaker and had the field coil connected in series with the B+. So looks like would be possible to get about a 3 db increase in power with only the extra electrolytic in the doubler circuit. We will also need a bit more iron in the OPT. So we have three cost items, a more expensive rectifier tube, an extra electrolytic capacitor, and a more expensive output transformer. An alternate way to gain 3 dB would be to use a push-pull output stage. This would obviously drive up the tube cost, and would require a few extra small Rs and Cs, however on the savings side one electrolytic capacitor could be eliminated, and the cost of the output transformer would probably be less. Using the 117Z6GT rectifier is not as elegant a solution but would work anyway. But you now need a heater dropping resistor. The 117Z6GT rectifier is totally inelegant! PP OP would no doubt deliver more power but brings with it other issues. These would include a CT OPT, phase inverter, extra octal socket, etc. Then there is the problem of enough heater volts. Two 50L6GT’s don’t leave anything for a tube rectifier. That could be run on a separate heater circuit. Or it would be possible to use 25L6’s instead on a 300 mA heater string. That translates to about double the heat, another problem. And you would need a rectifier capable of a bit more than 100 mA. Yes, the phase inverter and extra socket and associated small components add extra cost. My guess is that the cost of the output transformer doesn't increase as it does with the voltage doubler approach. Yes, a center tap must be added to the primary, however the air gap in the core can be greatly reduced vs. the voltage doubler single ended approach, meaning that the push-pull transformer will require less copper and iron resulting in a less expensive transformer. The 1:1 transformer secondary running the 12AU6 heater is tied directly to the –ve supply lead, so very little modulation of its cathode as otherwise may occur. While I considered the effect of heater to cathode leakage on a normal cathode resistor, and any leakage from the input circuit on the "isolation" RC, I completely missed the fact that the heater to cathode leakage current also flows through the isolation network and appears across the grid to cathode of the output tube. A push-pull circuit would mitigate this heater to cathode leakage effect to a great extent, potentially eliminating the need for the heater transformer in a push-pull version, another cost advantage for a push pull circuit. All academic now since this kind of technology would not get a pass from UL, CSA or anybody else! Ground the chassis and use a dual bobbin output and input transformer? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#10
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I was trying to come as close to the original as possible. Your scheme goes way off track. By the way, have you ever built a doubler of any kind & did objective measurements of its performance? How about showing us some real amps or tuners you have built from scratch (not a kit), your own design, not a copy of someone elses work. And please, with explanations of why you did certain things. Just checking if you are still able to ID the hot end of a soldering iron. Cheers, John S who is still able to ID the handle end of a screwdriver! |
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