DIY Audio Projects Forum

I'm still trying to decide whether to build a GM70 SE or PP. The PP sould be less expensive becasue a cheaper transformer could be used, but the SE might sound better. If I used parallel tubes in SE configuration I could almost double my power. B+ will be 1200 volts with each tube biased at 100mA in either configuration. The PP would use a 20K tranny (plate to plate), the parallel SE a 5K tranny, so the GM70 would always "see" a 10K load. Both configurations use 2 GM70 tubes per channel. The transformers for the SE will cost more, but it might be worth it.

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Stephen

If You have PP transf. 2 x 10 kohm, You can use for SE one section 10 kohm, but You can
dismantle transf. and mantle Ш and I /Ш from one side and I from another side/ and use
only one GM70. You cane make gape between Ш and I 0,15 mm /1 paper/

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/

One of the major problems with the GM70 is the high output impedance and the fact it's difficult to find transformers with 10K and 20K primary impedances that are designed to handle any power. Most 10K and higher transformers are very low wattage.

Edcor and Hammond (the two affordable choices in the US) offer little in the way of high impedance transformers. The best I can do is either change the reflected impedance by using an 8 ohm speaker on the 4 ohm tap (limiting the output power), or use parallel tubes to allow a lower impedance transformer to be used.

I think I'll stick with the PP SIPP stage since it appears to be unique! At least I can't find an example of anyone who has used the GM70 in a SIPP output stage using a high voltage CCS anywhere on the internet.. I can find PP GM70 amps, but not in the SIPP configuration. If anybody knows of someone who has built this please let me know. There MUST be someone who has done this, but if they have I can't find it posted anywhere.

Also, the Edcor PP transformers are a lot cheaper than the SE Hammond that I would need to replace them with if I went single ended. If the amp doesn't deliver in the sound department I'll sell the trannys and make an SET amp.

If it weren't for my 300B I would go with PP and not look back, but now that I've discovered the sound of the SET it's become a difficult choice, but I'll wait for my PP Edcors to arrive and build the PP.

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Stephen

SE sounds tonally differently from PP. Technically, SE tends to give more 2nd harmonic distoriton plus modulation of 120Hz HV rippling, that pleases many many ears with such sorta 'syruppy' sonic effect.

PP with cancellation of 2nd harmonic distortion & 120Hz rippling due to PP transformer primary winding, which give many ears less 'pleasing' effect. But this does not mean PP is not as good sounding as SE. It get music cleaner & more dynamic sound which pleases me better.

I've been listening to my EL-34 Vg2 level shifting triode-strapped PP for years, which deliver clean, dynamic & transparent sound that make me feel SE sound too 'syruppy' than real.

This impression is further stressed during an audition of a pair well-known brandname power monoblocks. ("Handcrafed in California") using 300B PP, a couple years back. We tube rolled 300Bs & found WE300B PP blew away the stock Chinese 300Bs big bigtime. It delivers music right without the characteristic sugarly colouration of SE.

c-J

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"If the facts don't fit the theory, change the facts" quoted Albert Einstein.

With my SE UL 6T9 tube amp I run a pair of KEF iQ30s. These are a single driver with concentric tweeters. The sound stage is pinpoint sharp and has reasonable depth.

The same with my latest KT88 SE UL. I "personally feel" this is another area where the SE shines.

Remember all our comments are subjective. But that is what forums are about. Individuals making personal comments.

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Projects:"retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp

For me it tends to depend on the speaker. With low excursion single voice coil speakers I tend to prefer a single-ended amp.

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]

Hi all!
I just wanted to pop in here and add my opinion on the AC vs DC vs SMPS on filaments. Now, I don't intend on arguing the possible sonic influences these various filament PS schemes may have. So don't try to get me to go there. I personally have never been able to discern any difference based solely on the filament PS. Most colouration I have heard has indeed been an inflection or reflection of the quality of the HV power supply, tubes in play, and other sources of inducted noise due to poor routing of signal wires. So no, I don't prescribe to the sonic argument in relation to filament PS. To me, the best argument for DC on a filament is to preserve filament (and therefore tube) life. Now one might argue that a SMPS is a pure DC, but it is not. It is a High Frequency modified and rectified AC signal. Some SMPS have very good final stage filtering, but they generally cost a little more and have garnered the "computer grade" label. For the purpose of filament preservation; any SMPS will do, but it is overkill. Do take caution if you're using a lower grade SMPS to keep the filament wiring separated from signal wires as much as possible. While you're not likely to actually hear the signal of the SMPS @ 30KHz + ( to my knowledge, no human posseses the capability to hear frequencies above 20Khz) you can indeed hear other noise that can be inflected from early stages - such as the primary rectification at mains in - should that noise be inducted in to the small signal lines of the amp. It is duely notable that there are some really cheap SMPS schemes out there that are in the human sonic range and they can naturally induce that high frequency signal in to amp signal lines. It will appear as a high pitch ringing or whine. If you want to use SMPS, go for it. I will use simple DC rectification and filtering. I have been able to acheive sonically quiet pure DC power supplies easily and cheaply. I don't use regulation unless VOLTAGE or CURRENT stability is an issue. Regulation should never be confused with noise rejection. The purpose of regulation is to maintane either a constant current or a constant voltage. Some regulators boast good noise rejection, but the simple fact is, they don't do it well. Again, in my opinion and for the purpose of filaments, it doesn't make a hill of beans difference, sonically speaking. So, since in my opinion power type on the filaments has no influence on sonic quality in an amplifier, why would I prefer DC over AC? To preserve the filament. That filament in a AC set-up is undergoing the same stress as a household lightbulb. It litereally is being turned on and off 120 times per second (at 60cycles, the voltage crosses 0v twice in every cycle [60 x 2 = 120] and changes current direction at every cross). It is a technically proven fact that a lightbulb will last far-far longer on a DC voltage than an AC voltage. The purer the DC the better the life span. So, for that purpose, an SMPS fits the bill just as well as a simple brute force AC-DC rectified PS. But if you want to believe that somehow that AC signal is inflecting some sort of pleasant colouration to your amplifier... Hey, its your tube and your money!
GOOD LISTENING!

Ps. Nice build sampleaccurite!

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

Thanks. The main reason I use SMPS is cost, ease of implementation, and weight. I just got some 12V 6A SMPS supplies on ebay for $9.99 and free shipping for a 6C33C OTL amp:

http://cgi.ebay.com/AC-110V-240V-Adapte ... 48370ff384

I expect they will supply 6 amps DC at 12 volts in addition to a large amount of high frequency trash. But they will be shielded from the rest of the amp and the output of each will be filtered before it leaves the shielded area of the chassis. I've use them on KT88s with the Oddwatt as well as 300Bs and I've yet to have a desire to go back and replace them.

I've read some interesting claims about DC heater supplys used for directly heated cathodes and the signal current induced in the filament from one side to the other by the difference in signal potential having an effect on the sound, as well as the non-uniform distribution of current (electron) flow in the tube. Considering the very large anode to cathode potentials of the GM70 it seems to me that this effect would be negligible, but not having the opportunity to listen to ANY amps with AC vs DC I have only math to show the differences in potential between one side of a cathode and the other are tiny compared to the anode voltage. HOWEVER - consider that the DC voltage across the filament is NOT always tiny compared to, and in fact may be a significant portion of the grid to cathode voltage. If someone claimed that AC heating sounded better than DC with a particular tube in a particular circuit, I could believe that. However, if it does sound "different", I have to wonder what happens to the signal when the electrons flowing in the tube originate from a wire that changes its polarity 60 times a second. Add to that the only reason 60Hz is used is because it's the most convenient frequency.

The AC/DC debate will never be settled. Whatever pleases you the most is what you should use. I just want to get my amps up and running and tweak later. Use a bigger power tranny than needed, shoot for a little bit higher voltage than required, then drop it as needed with power resistors to fine tune the B+. That's my lazy a** method anyway, but it has served me well so far.

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Stephen

I've occasionally wondered why we (the US) settled on 120VAC@60Hz and a major part of the rest of the glode 240V@50Hz. Are we weird, eventhough we started this whole electric grid thing, or is everyone else weird? I know why we settles on AC vs DC, I'm just curious on the particular voltage and frequency. If I learned this in electronics class, I don't remember it. That's been too many electrons over the jumper since.

Don't get me wrong, I was basically supporting your decision for using SMPS, or anyone else's using what ever current form they want. I just simply haven't heard any discernable difference with AC v. DC on filament. Maybe there can be case based on scope trace, but my ears can't hear scope trace, they hear audio. I learned old school, and old school weren't scope dopes. We used scope for signals not audible either due being to small or above hearing, like radio. Audio - best tool is ears, not scope. If it sounds good, then it is good. If it doesn't sound good, well maybe then break out the scope and find the source of the yuk and fix it.

I've got about 3 projects rattling around i my head right now that I would like to do. One project I have got all the parts for, namely the iron and glassware. The other 2 I haven't got much, maybe an extra 12AX7 here and there. And since the first project is a rather ambitious project, and this will be my first attempt to build a tube amp from scratch (I've done a few sand/chip amps = lower voltages), I am having second thought that maybe I should start something simpler, like a single-ended 300B or even a nifty little 12AU7 based PP I dug up. Add to the fact I haven't worked on a tube based system in a very long time. Gotta blow out the dust.

I saw another interesting tube in the forums that has sparked my interest. It's the 6C33. But what I've seen so far has had some serious power transformer concerns. It's interesting, to say the least. Maybe a challenge worthy of my desire... We'll see.

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

That's what those SMPS above are for - the 6C33C. With 6 amps output they should easily power the 3.3 amp heaters, one SMPS for each tube.

The power tranny I'm using will be two simple isolation transformers, 120v/120v, one for the positive power rail and one for the negative (120-0-120 volt, 2500mA). At 300VA each (600 VA total) I'll have a split supply with about 2.5 amps at about 240 volts before rectification.

After a PP (Oddwatt) and an SE (300B), I want to do something different, which is why I'm going to build a 6C33C OTL. My only concern at this point is the use of a lot of negative feedback to make the output impedance low. How gracefully does an OTL with 25-30dB of feedback clip? My guess is it will hard clip similar to a transistor amp, although I'd like to find out. BTW, I found the 20 turn, 1 watt cermet bias pots, so that's the last of the parts with no substitutions. I figure the OTs save about 20 lbs., and the heater supplies are at least 5 lbs lighter than using traditional filament transformers, so the amp should be very lightweight for a tube amp with its power rating. It's just going to get very hot.

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Stephen