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PostPosted: 29 May 2010, 06:35 
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If a stereo SRPP driver is desired from only 2 tubes (1 per channel), and a high voltage swing is required, the problem of heater to cathode voltage crops up when using a dual triode.

However, if the tubes are shared by the two channels, with one tube functioning as the bottom triode for each channel and the other tube functioning as the top triodes for each channel, the tube heaters can be biased so as to avoid exceding the heater-cathode breakdown voltage.

A 6SN7 SRPP using only two tubes should be able to easily drive GM70s.

There is the possibility of slight interaction between channels with tube sharing, but my guess is it's preferable to a simple CC triode gain stage.

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PostPosted: 29 May 2010, 21:23 
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sampleaccurate wrote:
Here's the layout on a 17 x 14 x 4" Hammond chassis:

QUESTION: Notice the orientation of the plates and cathode filament wires. They are in the same plane, "facing front and back" if you will, as opposed to being in parallel planes "pointing at each other". Is this the correct way to orient the tubes to minimize interaction? Or do I have it backwards? I know the KT88 specifies a certain orientation but the description makes no sense to me. It would seem to me the direction of electron flow would be perpendicular to the plane of the cathode, and based on that assumption the orientation should be as shown, but I'm not sure. :confused:

As always any help or suggestions would be appreciated.

Image

Does anybody know the answer to the question about the plate orientation? Does it even matter?

After the KT88 build, I'm going to make some printed circuit boards for the Giganotosaurus. It's the only way I'll ever fit it in the case, and I now have a feel for how the components are arranged when actually built as opposed to a schematic on paper.

I need to know how I should orient the plates so I can design the circuit board to match the pinout of the GM70 and keep leads as short as possible.

Thanks for any help.

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PostPosted: 31 May 2010, 15:07 
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sampleaccurate wrote:
any help or suggestions would be appreciated.

Hi.

If you ask me, I woud strongly suggest you to take out the SMPS as far away as possible from yr amp
to lower the airborne RFI transmiitted from the SMPS. Use a long power power cord to pass the DC to the tube heaters inside the power amp.

Check out my post on my new 6W DC mini iron heated by a 6.5V2A SMPS - airborn RFI from the SMPS.

c-J

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PostPosted: 31 May 2010, 20:44 
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cheap-Jack wrote:
If you ask me, I woud strongly suggest you to take out the SMPS as far away as possible from yr amp
to lower the airborne RFI transmiitted from the SMPS. Use a long power power cord to pass the DC to the tube heaters inside the power amp.

Check out my post on my new 6W DC mini iron heated by a 6.5V2A SMPS - airborn RFI from the SMPS.

There's one more thing about an SMPS that can be important other than the RFI. The capacitance to ground is relatively high, around 1000-2000pf. Im most designs this won't matter, but in a DHT with cathode bias, or in a design where the heater is tied to the cathode in some way, an SMPS can screw up your circuit.

I had this concern with using an SMPS for a 300B heater, but, apparently, and according to PSPICE sims, the cathode of the 300B can handle the capacitance.

How about I encase the SMPS units in mu metal, and put caps on the outputs before they exit the mu metal enclosure? That will work, or at the very least greatly attenuate any RFI.

I certainly don't dispute these SMPS units put out RFI trash. On my 300B I attempted to bypass all the non-DC components to ground with caps. That seemed to clean most of it up.

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PostPosted: 31 May 2010, 21:00 
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Hi Stephen,

well with those SMPS units you have to get lucky, I suppose. The only time I tried that, I had a very dirty unit making cleanup almost impossible, so my ex[perience with SMPS is as bad as yours is good, maybe I should try once more using a different unit.

To enclose the whole psu in metal is a very good idea, I got myself just for that very thin copper sheets, easy to bend and solder, i will make sub chassis's for everything in my next project. (Whatever that may be)

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My DIY Audio Projects:
- Single-Ended 12B4A tube amp with ECC802S driver
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PostPosted: 31 May 2010, 21:45 
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Bennybuis wrote:
well with those SMPS units you have to get lucky, I suppose. The only time I tried that, I had a very dirty unit making cleanup almost impossible, so my ex[perience with SMPS is as bad as yours is good, maybe I should try once more using a different unit.

To enclose the whole psu in metal is a very good idea, I got myself just for that very thin copper sheets, easy to bend and solder, i will make sub chassis's for everything in my next project. (Whatever that may be)

BTW, by contrast, a 6.3 volt heater winding on a transformer I tested had about 30 pf to the primary coil. So if you want to power a DHT, if the SMPS is to much capacitive load you can use AC or rectified AC from a low voltage transformer winding.

From what I've read powering the heaters using AC from windings on the same power transformer that powers the amp's main rectifier will induce switching noise from the PS diodes into the heaters. What's worse, that or a SMPS? Do you use a separate tranny for the heaters for amps with SS rectifiers?

It seems to me the best way to power heaters - with no regard for cost - would be to use a separate transformer, rectify the AC to DC, regulate the voltage, and filter it. It's a low capacitance (to signal ground) way to provide hum free power to the heaters.

Sounds like you might have gotten a bad SMPS. I've had great success with them.

My GM70 directly heated cathodes will need to "drag" around the 1000pf or more of capacitance that these SMPS units will place on them. The 300B can do it. A separate supply with a transformer for the GM70 tubes would be huge and expensive. I hope the SMPS works.

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PostPosted: 01 Jun 2010, 09:22 
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sampleaccurate wrote:
How about I encase the SMPS units in mu metal

Hi.

Yes, the only way to enclose ariborn RFI is to encase 100% the SMPS with a mu metal case.
Make sure you test it with an AM radio right beside the metal case to detect any RFI leakage.

The AC power cord & the O/P DC power cord of the SMPS MUST get RFI ring suppressor snapped on to stop RFI getting into yr power amp & other audio components via the powerlines. Any shunt RF caps (ceramic caps) are not good enough.

The above should be good enough for IDH tubes. But you now are using DHTs.

My very concern of using SMPS to heat DHT tubes is the tube filamments also act as signal path. Technically it is impossible to be get the music signal 100% free from RFI contamination in the DC supplied by the SMPS.

I know you want to save a bundle & hassle to get analogue PS to heat the 300Bs. But you are now opening up another larger can of digital worms. Good luck, bud.

FYI, I am using 6V SLA cell (rechargeable by a 7.5V4A SMPS) to heat IDH tubes for my phonostages. To ensure zero RFI contamination (which is very unluckly as the SMPS is always switched off when the battery is heating the tubes), I have the entire heater circuit floated from the amp circuit. So the unlucky RFI contamination via the signa reference ground can be avoided.

c-J

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PostPosted: 01 Jun 2010, 09:37 
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cheap-Jack wrote:
sampleaccurate wrote:
How about I encase the SMPS units in mu metal

Yes, the only way to enclose ariborn RFI is to encase 100% the SMPS with a mu metal case.
Make sure you test it with an AM radio right beside the metal case to detect any RFI leakage.

The AC power cord & the O/P DC power cord of the SMPS MUST get RFI ring suppressor snapped on to stop RFI getting into yr power amp & other audio components via the powerlines. Any shunt RF caps (ceramic caps) are not good enough.

The above should be good enough for IDH tubes. But you now are using DHTs.

My very concern of using SMPS to heat DHT tubes is the tube filamments also act as signal path. Technically it is impossible to be get the music signal 100% free from RFI contamination in the DC supplied by the SMPS.

I know you want to save a bundle & hassle to get analogue PS to heat the 300Bs. But you are now opening up another larger can of digital worms. Good luck, bud.

FYI, I am using 6V SLA cell (rechargeable by a 7.5V4A SMPS) to heat IDH tubes for my phonostages. To ensure zero RFI contamination (which is very unluckly as the SMPS is always switched off when the battery is heating the tubes), I have the entire heater circuit floated from the amp circuit. So the unlucky RFI contamination via the signa reference ground can be avoided.

I think your concerns are valid about the DHT and the SMPS becoming part of the signal path. However, I don't see RFI as an insurmountable problem that requires batteries capable of supplying 20 volts at more than 15 amps total while I listen to the amp. I'd love to see some pics of your setup - it sounds quite elaborate and I'm sure it sounds awesome. Proper shielding, with chokes and capacitors on the input and output should adequately reduce any RFI to acceptable levels, but if you feel differently I won't argue with you.

I think your bigger concern should be the high capacitance and the "load" the SMPS represents to the cathode of tube. It's not really any different than putting a 2000pf cap from your cathode to ground. That's where you're likely to run into problems IMHO.

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PostPosted: 01 Jun 2010, 11:36 
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sampleaccurate wrote:
RFI as an insurmountable problem that requires batteries capable of supplying 20 volts at more than 15 amps total

Hi.

No, I never suggested YOU should use batteries to heat up power tubes. I won't but I only used batteries for pre-amps for logical reasons.

While tons of 300B & the like DHT DIYers used AC heating & DC heating with sand diodes, they should also know SMPS is an easy going way to get things done. You should rethink why they are not using it?

Anyway, do what you think you should do to suit you. I am here just to point out a few things one shuold know.

c-J

PS: my Dynaco ST-70 is still AC heating but I installed choke+AC cap filters for each channel.

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PostPosted: 01 Jun 2010, 14:08 
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Updated schematic of the GM70 oddwatt - Giganotosaurus.

Image

All heaters are SMPS. Each GM70 has a dedicated floating 20 volt 4 amp SMPS across its cathode that's filtered with multiple caps to reduce any "non-common mode" RFI. Any common mode RFI on the SMPS can be reduced with chokes, or, with bypass caps to ground if the SMPS is used for indirectly heated tubes.

The BUX85G is rated at 450 volts Vce and has 20 watts of dissipation capability with the proper heatsink.

If I could only get my hands on a PSPICE model for a GM70, or even an 845 would suffice to approximate the response. I'll have to be content to model the driver stage and place a dummy load on it to simulate the GM70 and design it for output voltage swing. The GM70 stage is simple enough so that it really doesn't need modeling - the transformer model isn't available anyway. The biasing is simple with the CCS.

The only real concern I have at this point is tube balancing. If the tubes won't balance in the SIPP, I'll build a phase splitter, use fixed bias on the grids of the tubes and ground the cathodes. That has to work, but I would like to use the SIPP output if possible.

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Last edited by sampleaccurate on 01 Jun 2010, 14:52, edited 1 time in total.

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