New Project: 'Junk Parts' Amp build - Voltage / Bias question

Discussion in 'Tube Audio' started by Wharfcreek, Jan 10, 2019 at 11:29 AM.

  1. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    So you play around with this stuff long enough, and pretty soon you see stuff on the shelves and in the corners where you say: "..I"ve GOT to do something with that some day!!" Well, that day has come...... at least for some of it. I'm starting a build on an old Motorola Chassis that I've put an 'aluminum deck top' on, and have planned as using an old Channel Master 6600 power transformer, and a pair of Edcor PP 15W 7.6K PI @ 8 ohm output transformers. To be 'tube rectifier, with 12AX7 drivers and EL84 outputs.

    First, I don't have a schematic for the 6600, so I'm not exactly sure what it's outputs are. I believe the unit used a 5AR4 rectifier.....so I'm thinking it may have a 5V filament current limit at around 2 amps. So, to be on the safe side, I'm sticking with a 5AR4 rectifier. Since I don't know how Channel Master configured it's power supply, I'm going to have to play with that some as well. I believe the 6600 used 4 6V6 output tubes, so I'm only guessing that this will all work out. But, I'm thinking that I should end up with a B+ right around 300 VDC, give or take.

    My thought is that in using the EL84/6BQ5 output tubes, I should be able to run those into the Edcor output transformers even though they're a 7.6K Primary vs the more desirable 8K. But, because of the PI discrepancy, I'm wondering if anyone has any advice for me on the issue? I'd planned to model the thing somewhat on the same kind of final output section one might see in a Magnavox 9300. In fact, even the front end may look like that to some degree, as the 6EU5s can easibly be replaced with 12AX7. I'd also been looking at an old Fisher Custom Electra power amp schematic..... which also used EL84s, non - UL, and has a 12AX7 front end. The main thing is that the amp has to stay 'cathode biased'.....unless later EFB'd....which will only happen if the results of standard cathode bias prove rewarding enough to merit further effort into the amp. But, just wondering if there might be some 'target' voltages I should shoot for with this power supply section, and if the 7.6K PI should be considered when setting the bias level? Any thoughts or suggestions would be appreciated. I'll post a pic or two shortly!! Many thanks, Tom D.
     

     

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  2. FlaCharlie

    FlaCharlie AK Subscriber Subscriber

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    Tom - here's the Sams for the 6600. It does use a 5AR4 but has 6BQ5 outputs.

    I'm not familiar with the Edcor OTs you mention, and I'm probably not knowledgeable enough to offer much advice, but I'm curious what the "PI" designation is all about.
     

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  3. gadget73

    gadget73 junk junkie Subscriber

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    You could do a lot worse than using the ChannelMaster schematic as your basis. Direct coupled split load inverters minimize the possible spots for phase shift and it removes a cap from the signal path. Its also generally a better performing inverter compared to the paraphase that the Magnavox uses. The only down side is heater-cathode voltage differences but you can elevate the heater supply to keep that reasonable. Quick and simple way of getting there would be to tie the hum pot center wiper to the cathode of the EL84 tubes. The cathode should be a +12v or so, which would reduce the H-K voltage by that much.

    EFB is perfectly do-able, should be a near match to the Magnavox 9300 output stage at that point. Basically ditch the cathode resistors and the balance pot and sub the EFB regulator. If you want to do individual bias adjustments, I believe Dave covered that in either the SCA-35 or ST-35 with EFB article. The two EL84 type amps I have with EFB have just a single adjustment and I run matched tubes. It makes life easier.

    The only real fussing is the feedback and HF stability stuff, and thats going to depend on the output transformer. To keep the same level of feedback as the stock design, the 47k feedback resistor would become a 33k. The cap you'd need to fuss with based on how it acts. Same with the 47pf cap on the one output tube grid.
     
  4. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    i think PI is primary impedance for the OPT. PI is usually meant for phase inverter or private investigator in law enforcement circles.
     
  5. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    Wow!! Didn't expect that!! Many Thanks FlaC!

    'PI' refers to the 'Primary Impedance' aspect of the output transformers. This is relatively critical as this is what presents the load back to the output tubes themselves. As different output tubes work better at different load levels than others, it's somewhat critical to 'match' the Primary Impedance of the output transformer with the tube you're using. I might add that 'secondary' impedance is a factor here as well.......as the transformer's primary impedance is somewhat of a function of the secondary impedance. So, I start by choosing my secondary impedance.....and since all my speakers are 8 ohms, that's the secondary impedance I go with. Then, looking at EL84 output tubes, these like to have an 8K primary impedance when running in push-pull. If running in a Single Ended design, they are better matched to a 5K PI output transformer....again, depending on what the secondary is going to be. If you were to go to Edcor to 'order' an output transformer, you could order a 4 ohm secondary with a 5K primary, or an 8 ohm secondary with a 5K PI, etc. Likewise with the PP output transformers, but now you'd be looking at ordering an 8K PI @ 8 ohms if going with EL84 output tubes. So, going back to my transformers....they're simply not 'ideal' in that they're 7.6K PI vs 8K. In reality, this may not amount to being too significant a difference, because 'speakers' are not 'exactly' 8 ohms.....and, when operating, they can be all over the board. So....this may be negligible. But, still worth it to me to 'check with the experts' here on AK to see if anyone has any specific advice about it. This place is a wealth of information, not to mention just full of great people. So, again, thanks for the Schematic on the 6600......and hopefully I've cleared up the PI thing. Attached are a few pics of where this amp stands at this point. Pretty obvious why I chose to put the aluminum sheet across the top. Now it's just a matter of cutting in the tube socket holes, drilling the mounting screw locations, and deciding if I want to put the on/off switch and pilot light on the top deck or not. I think I do! Also have to mount the fuse holder and RCA jacks on the back...but then it will be time to start soldering...........

    Tom D.
     

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  6. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    With the PT from the channel master meant for stereo PP el84 , building a similar amp should be no problem. Any of the classic PP el84 circuits will do. Scott circuits use pentode/triode driver/splitter tube. Fisher like all triode 12ax7 for that duty. EICO likes 12dw7 or a dissimilar triode. Any of these circuits will work with what you have.
     

     

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  7. Shadowdog

    Shadowdog Super Member

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    Some commercial EL84/6BQ5 PP amps have used less than 7.6K primary and from what I've seen on the forums that little difference shouldn't matter.

    Near 8K favors fixed bias from what Dave Gillespie has said and if you had 10-11K cathode bias.

    Dave used 7.2K Heathkit AA-100 on a EL84 PP amp he built a few or so years ago.

    You might have to fiddle with the feedback for different transformers with nothing to do with the primary impedance .
     
  8. 6DZ7

    6DZ7 Super Member

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    Don't think I'd parallel 2 40uF caps off the 5AR4 though. Put a C-354 between them.... like, duh, right... But what was Channel Master thinking? ... engineers...
     
    Last edited: Jan 10, 2019 at 1:12 PM
  9. FlaCharlie

    FlaCharlie AK Subscriber Subscriber

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    Tom - glad I could help. Yeah, I'm familiar with primary impedance of OTs and all the rest. For some reason I thought it was part of the model number of the Edcors. I guess I didn't read closely enough to see what you were asking. When I see an OT described as 7.6k I assume that's the primary impedance.

    I have numerous vintage PP 6BQ5 amps and their OT specs vary quite a bit. Tubes seem to be relatively forgiving in that regard.

    Just out of curiosity, I pulled a few Sams out from different 6BQ5 amps and here are some of the OT specs:

    Bell 2440: 9k
    Channel Master 6600: 5.2k
    Channel Master 6601: 6.4k
    DeWald P-1400: 5.5k
    Eico HF-81: 7.5k
    Grommes 40PG: 9k
    Magnavox 9301: 8k
    Pilot 240: 7.4k
    Pilot 244: 7.3k
    Stromberg-Carlson ASR-433: 5.7k
    Sherwood S-5000: 7.8k
    Voice of Music 1428: 6k
     
  10. s-petersen

    s-petersen Scott Subscriber

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    I'm doing almost the exact same build for someone, using Dave G's 9300 schematic, and maybe a touch of Gadget's as well with EFB® .

    2 back-s.jpg

    1 front-s.jpg
     
    Last edited: Jan 10, 2019 at 5:53 PM
  11. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    Scott............... I like the blue!!! But, I gotta ask as those look like all 12XX tubes in the amp: Am I missing something?

    Charlie, again, that some great info! I think the word 'forgiving' is hopefully correct. I think I can get by with this 7.6k PI, though Randy has mentioned the aspect of 'fixed' vs 'cathode' bias as being a potential issue.

    6, I agree.....that looks a little 'iffy' for a 5AR4 filter set-up. Oddly, I was thinking about this earlier this afternoon. Fortunately, I had a Choke arrive in today's mail, so I will definitely use it. I think it's that 156 Hammond Choke.....good for I believe 230ma. Should be fine in this amp. Also, I generally run a 10uf / 500 v off the pin 8, then into a 'dropping' resistor as needed to get a proper voltage. That's been mostly working with Class A stuff. I know it's not a good idea to put the resistor into a AB design if it can be avoided. So, I'm going to use the choke in it's place and then go to 47uf / 450 V and see where I'm at after I try building the output part of the amp and see how the PS operates when the tubes are all loaded properly. This saves some time in the long run in 'adjusting' the rest of the PS voltages. Also, if those two 40uf caps shown in the 6600 schematic are reduced the way I do it, I may get an actual drop in PS voltage to closer to 300..which is rather my 'target' for this build. But, I suppose that can deviate some without too much adverse affect.

    Thain, I'll look more at that 6600 schematic.....but having no prior experience with that one at all....... I'm just looking to stick with the 'familiar' if at all possible. However, you and Primo are right about the Phase Inverters stuff. I going to try to pick that part of the circuit as wisely as possible. Actually, I have an old Rowe Juke Box amp schematic that uses one 12AX for each channel.....and I've ganked that part of that circuit in the past and used it for other builds as a 'driver' and it worked well. Also EL84 amps, but also on one 7591 amp build.....as it was originally designed around 7868s.....and it works well. So....still unsure. PS and Output sections first...then 'onward' from there.

    Scott, is that blue paint in a can?.........lol '.... it's electric....'

    Thanks guys!!! I really appreciate the comments. Very helpful going forward.
     
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  12. s-petersen

    s-petersen Scott Subscriber

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    Yes, they were just standins as I was mocking it up, The 6BQ5's are in it now, I went with DC heaters in the 12AX7's The Paint is Rustoleum 7251 cobalt blue metallic. It's decent paint this time, The copper takes forever to dry, this was pretty fast.
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    If the Fisher you're speaking of is one of the 440/460/480 type amps, honestly its not that different from the Channelmaster circuit. The one big difference is that the Fishers used several tube heaters as a cathode resistor. It ends up with a resistor size that is considerably larger than is needed to bias the tube normally, so they have a resistor network that takes voltage from the cathode and feeds it into the grid. It also needs extra plate voltage to make up for the voltage lost on the cathode side. It works, but without the extra plate voltage you'll lose output power.
     
  14. FlaCharlie

    FlaCharlie AK Subscriber Subscriber

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    You can safely use up to a 47uf cap after a 5AR4 so there's no absolute need to go as low as 10uf unless you're doing it to reduce the B+. That said, in practice in might depend whether you're using an old stock American or European 5AR4 or a current production. If current production, you might consider doing the so-called diode mod which Eli Duttman has posted about many times over the years.

    Using the choke would be a good idea but I would note that all the highly regarded vintage PP 6BQ5 amps use dropping resistors in their power supplies. Off the top of my head, the only one that used a choke was the relatively lowly Magnavox. One of the most highly regarded PP 6BQ5 amps, for example, is the Pilot SA-232. BTW, it uses a 6k OT. Here's a section of the Sams for that amp:

    Pilot 232.jpg
     
  15. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    FC, that looks good! Maybe that's the circuit I should use! Note the (I believe) R85. It's a bit hard to see....but I think that's it. It's a 270 ohm resistor in line to one of the two screen feeds. Looks like it has some kind of 'dotted line' that maybe indicates some kind of 'bypass'? Got any info on that? Looking at the voltages, I think the 6600 PT is a bit less on the HV side....but perhaps if I go with a 47uf cap right off the rectifier tube socket, then to the choke, then a 60uf cap..... I might get close. I think the 6600 schematic shows 330V.....and this shows 350. I think if I change the 6600 PS design I could squeeze another 10+ VDC....which would get me close enough. Thoughts?
     
  16. FlaCharlie

    FlaCharlie AK Subscriber Subscriber

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    The dotted line with the x's on it, and a note on the resistor parts list, indicates that R85 is not used on some versions. If it is used there should be another one going to the screen of the other tube. Some type of screen stopper would probably be a good idea.

    As far as voltages go, remember the ones listed on a Sams are taken with 117vac from the wall and +/- 15% is considered to be in spec. Obviously, you'd want to be closer to the listed voltage, which you should be using modern wall voltages. The other wild card would be the DCR of the choke, which would replace R94, the 100 ohm 10w. A few volts either way in the B+ isn't critical. Just be sure the plate dissipation of the 6BQ5s is reasonable.

    If you copy this design, I would suggest that you use something other than a 12AU7 as the input tube. Apparently it is not very linear so not such a good choice in this situation. I'm using a 12BH7 in mine, which is pin compatible with a 12AU7.
     

     

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  17. kward

    kward AK Subscriber Subscriber

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    I'm listening to my refurbed Motorola PP amp as I write this. Its my daily driver and only amp i have until I finish my SE-UL amp. My motorola is 6V6 output tubes though.

    Considering your output stage, with the tubes being pentode connected, the primary impedance is less critical--anywhere between 7K and 8K will be fine, IF you don't mind adjusting the screen voltage so the tubes are loaded properly. Proper screen voltage would depend on center tap voltage obtained from your power supply. My gut says you will want maybe 80% of the center tap voltage for the screen voltage for now, without EFB. A more exact calculation could be provided if you can state with some accuracy the center tap voltage.
     
  18. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    FC, in looking at that schematic again I now see what you're referring to with the 12AX and 12AU. Honestly, I missed that originally.....I thought that might be some vestige of the 'pre-amp' section and that all that was necessary was the 12AX portion. I'd mention here that I've planned the amp with only 2 driver tubes.....so I'm going to need a driver section that consists of just that. Be it a 12AX, AU, AT, a 'dis-similar twin-triode like the 7247, or a triode / pentode like the 7199, 6GH8, or even a 6AN8. I guess I'll cross that bridge when I get to it. For now, I just want to get the output tubes lit and 'powered' such that it's drawing current. Once that's done I'll worry about what driver design to incorporate. Given this 'glitch' with the need for the AU (or BH7 as you recommend).... I'll probably go back to either the Rowe driver or something else. The Fisher schematic I have uses one 12AX7 per side...... but I don't recall the model. I've seen what appeared to be this same amp with both 12AX7 and 7247s......so, I guess there are some 'Fisher' deviations that one does need to be careful about.
     
  19. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    K, I hate to be the dummy here....but can I calculate this CT voltage with the PT 'not' in the circuit or being 'loaded'? I'm not sure exactly what you're looking for here. I don't know that I've run into this reference in the past.....but probably just my ignorance. I appreciate the consideration though. And, if I'm thinking about this correctly, you're saying that if the voltage going to the CT on the output transformer is, ie, 320VDC, then I'd want about 80% of that on screens? If this is true, then is there any consideration for the loss of voltage through the OT itself?
     
  20. kward

    kward AK Subscriber Subscriber

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    Yes.

    The DC loss across the OT is negligible. It will be 3 to 4 volts.

    You can use PSUD to figure the CT voltage, or just back of napkin calculate it with the following procedure. Take your loaded PT voltage for one leg, multiply it by 1.3 (not 1.4 as is normally done so as to account for PT loss to heat), subtract 35 volts drop for the 5AR4. This will be the B+ voltage (voltage at first filter cap). Now subtract another 10V or so for the DC loss in the CLC filtering to get your CT voltage. That value will be within 10% or so of the actual voltage.
     

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