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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 04 Nov 2008, 20:43 
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Thanks Bruce. I will give this a go with 6SL7 and 6V6.
gofar99 wrote:
With a SS supply it is possible to have the full B+ across the tube before it is warmed up. I like to include a 30 second (more is fine) delay before applying the B+. Additionally it may not be obvious but the resistor values and capacitance values in the last two sections of the power supply filters were chosen so that they charge up just in time for the tube to warm up. The B+ thus does not exceed the H-C rating at any time. The values would need to be adjusted for other tubes and current drains. Once charged up the filters need to remain at the correct B+ when the circuit is working. For the 6SL7 this is in the 1.5-1.7ma range.

I was thinking of putting a switch on the B+.

I'll start a new thread once I get going as I'm sure I will have a lot of questions! :)

Cheers

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PostPosted: 05 Nov 2008, 11:21 
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Bruce,

I'm eyeing the Odd block(s) for my next project. I have a trivial question, what is the size of the Hammond enclosure you used? I've got a pair Hammond 1451-22 amp chassis waiting for something to be done with them and I'm trying to determine if they'd be about the right size. I suspect height for the transformers would be my issue- Anyways, thanks for putting this out there, I'll let you know if I get solder flowing - Jason

EDIT: Bonus questions, did you happen to try a pentode/triode mode switch? I'm just a novice here, but it appears to me it would only require switching the screens off the transformer taps and connecting them to the plates with a current limiting resistor. I'll also probably be trying either 12AU7s or 12AX7s in the driver stage simply because I have a bunch on hand.


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PostPosted: 05 Nov 2008, 15:47 
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Hi JayW, I don't recall the chassis part number however they are the 10X12X2 chassis. The Hammond covers will fit, but I am not using them because of heat build up. The copper cage is a utility bin cut to size. They came from "Lowes" in the kitchen section and are around $6.

I did not want to go to triode mode although it is possible. Two reasons, one the output would be lower and two the drive requirements would be higher. I tried both 12AU7s and 12AX7s and neither sounded as good in the circuit (I did adjust the component values to bias them properly). A 12AT7 was better, but the 6/12SL7s and 5751s were the best sounding. Given the choice of tubes you have the 12AU7 would be my pick. You will have to adjust the bias to about 5-6 ma through the SRPP and accept lower gain (15-17 vs 40-50) in the SRPP. For a KT77 version it would be OK as they will work with about +/-15 volts of drive the KT88s will not as they require closer to +/-30. The earlier project using EL84s will however work with 12AU7s fine.

Good listening
Bruce

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PostPosted: 26 Nov 2008, 22:56 
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Hi Everyone, Just an update. The OddBlocks have about 200 hours on them now and sound great. I have had no problems with the amps. I have had a number of questions regarding the 6NO30 delay relays though. They are optional and you can use any method you like to delay the B+ while the tubes are warming up. A simple switch would do. Just wait about 30 seconds and turn it on.

Another suggestion from a diyer was to use DC only on the SRPP heaters and use AC on the KT88s / KT77s. This can make it easier to build, especially if you rectify the 12 volts on the heater windings and filter and regulate it. The 12 volt SMPS is then not needed. I use them as they are a cheap, good solution to getting 12 VDC. Remember to not ground any of the SRPP heaters as the tubes may fail. Still use the 1/3 B+ bias tap though. If you get the 12 volts DC off the same winding as the AC for the outputs don't ground them either. I plan to try the DC only on the SRPP concept on one of the earlier amps (the Oddwatt225) in the near future. AC on all tube heaters was reported to increase the background hum enough so it was audible at the speakers. I hate hum so this is a no go for me.

Good listening
Bruce

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PostPosted: 03 Dec 2008, 16:36 
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Hi Everyone, Just an update. I stumbled upon a pair of Klipshorn KG4s for a song and put them on the system. They are quite efficient and the KT88s really drive them well. If you need it loud, this is the way to go. If you have anything resembling a normal sized listening room ( say under 60 cubic meters) then the levels can easily go up into the hearing damage area. So, knowing that the amp can handle several kinds of tubes, I reset the idle current to 125ma (a pair) and stuffed in the JJ KT77s. The KT88s BTW were still biased properly after nearly 200 hours of use - less than 1.5 ma off center on one pair and right on for the other pair (at 180 ma per pair). There was none of the drift problem that occurred with the EH KT88s. Anyhow, the sound of the JJs is as I remembered it. Not as forceful and robust as the Gold Lion 88s, but more delicate and apparently more detailed. Still plenty loud. 18 to 20 watts per channel vs 25-30 is in audio sound levels not a lot. My recommendation is that if you have fairly efficient speakers go for the JJ KT77s. In the best spirit of DIY save about $125 on tube costs.

Good listening
Bruce

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PostPosted: 15 Dec 2008, 21:37 
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Hi Everyone, An update. The amps have probably 400 hours on them and are still perfect. Kits are still in the plans and about 2 months off. Getting such a project off the ground is not as easy as it might appear. Custom steel silk screened chassis, Edcor iron, custom PCBs and such are in the works. Price and source (either direct or through a tube dealer) of the final kits is still being worked out. Maybe both. Enough said, I don't believe in advertising in a forum. It's just that I have been getting a bunch of questions and hope this answers them for you all. Pester me again in January if I don't post an update.



Good listening
bruce

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PostPosted: 18 Dec 2008, 10:50 
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Hi Bruce, I've been following your designs while doing allot of research, I'm seriouly thinking of building a variant on the "Odd Blocks" but, I'd like to go with a common power supply, do you have any advice?


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PostPosted: 18 Dec 2008, 22:51 
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Hi Ward, A single power supply can be built for a pair of amps. I have one such variation now. There are a few considerations though. First the supply must be able to deliver around 500ma at 450-475 volts. Second if you go all DC on the heaters you will need about 5 amps at 12 volts. Not impossible to do, just a bit more costly than two cheap SMPS supplies. One builder used DC only on the SRPP heaters, this cuts the demand for DC to nearly nothing. I plan to try this (one B+ source, one SMPS to the SRPP heaters only and AC on the KT88s) when I get some new transformers in the mail ( a week or so away). There are a few other potential problems as well. The cables connecting the amps need to be able to handle the current and voltages. Since you will probably need to connect and disconnect them from time to time you have some sort of jacks and plugs. I used octal sockets and plugs. The problem here is that you can have live dangerous voltages on the pins of the jacks. I also found that I needed to put an extra section of filtering inside the chassis of the amps to get solid bass. If you look at the Oddwatt225 circuit as a guide you can break the filter chain after the second cap and place duplicates of the rest of the string inside the amps. You must change the value of the final resistor in the filter chain to twice the original to maintain the proper voltage on the SRPP. Another problem is the possibility of ground loops through the power supply grounds. I did not have this problem with mine, but it could occur. Equal length wiring can help here. Hope this helps, if not holler again.

On the availability of kits...... There will indeed be PC boards. Separate ones for the PS, active circuitry and one for the CCS, output and feedback sections. Why three , it makes it easier to build and a lot less costly to implement. Each chassis is 8X12 inches black powder coated, pre-punched, stenciled steel.

The target for the first batch is February.

Good listening
Bruce

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PostPosted: 27 Dec 2008, 06:32 
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Bruce,

I initially read about this interesting cct at the DIYparadise site, then found that you have taken the ball and run with it. I have a pushpull 6L6GC Chinese stereo amp that I would like to try the output stage principles on, it is point to point wired so easy to modify (compared to a PCB-based design). If possible, could I use a part or parts of the existing preamp cct to save major mods in the first instance? Maybe drive the output pair of valves from the first half of the 6N1 only? Attached the current schematic below.

This amp worked well for a while with the 6P3 (Chinese 6L6) valves, good bass, sounded excellent at times, but a bit too close to a tranny amp sound at other times. Then it would occasionally blow a fuse, then one valve flashed up, and the fuse blew again, so I bought some EH EL34's, they sounded very strong in the midrange but no good overall. In hindsight this was not the EH EL34's fault, as there was probably a fault in the amp. I then got some TAD 6L6's and put them in, still didn't sound too good, and one of them flashed up and the tube was white inside with lots of white ash inside it after the amp was turned off, or the fuse blew, whichever happened first. All this time I had had a few fuses blow, several weeks apart, amp in daily use, so decided there was an intermittent fault and temporarily retired it. Recently I did a lot of inspecting (with x 2.5 reading glasses) to look for visual clues, and drew up a schem while I was at it, and tested most of the resistors and compared resistances between similar points on each channel. I could not find anything wrong. The amp uses fixed bias and I was suspect on the bias pots as they were the only component in the whole amp that looked cheap, and when I was biasing for different valves they were difficult to adjust, so process of elimination, remove fixed bias and give it cathode bias. After 2 days running 2 x TAD 6L6 and 2 x 6P3 it is sounding quite good and hasn't blown a fuse - yet...So maybe now is a good time to move on ahead and try the Oddwatt cct, the EH EL34's survived this amp with no casualties and sound great in an SE amp I am running, so maybe I could do the Oddwatt conversion and run the EL34's while still on holidays and have the time. As you can see from the schem the bias is proly on the cold side, I am having difficulty matching 1960's datasheets "typical operating conditions" to modern day UL connected pentodes, am still a newbie. Sorry for the long rave.
Attachment:
Ming Da MC34B schematic mod 1.gif

Ian.


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PostPosted: 27 Dec 2008, 17:26 
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Hi Ian, it would be easy to convert and the tube choice is fine (6L6GC). From your posting, I would agree that bias failure was the cause of the flash and meltdown. Without the control bias the tubes will draw everything the PS can deliver. The results are often dramatic. I have used 6L6GCs in the the Oddwatt arrangement and they (JJ) work fine. I would initially set the current at 125 ma for a pair (LM317 and 10 ohm resistor). If all is stable then go to 150. My experience using the 6L6GCs was that they have a very tube-ish sound. Similar to the Gold Lion KT88s but not as detailed. The JJ KT77s I have used have a more detailed sound. If you like rock, and such the 6L6GCs would be my pick, if you listen to music with lots of detail (solo female vocals, stringed instruments) then the 77s are my choice. The Gold Lions fall in between but are quite a bit more costly.

If you need data on current tubes go to Frank's Tube data on the web (google it). Also google the tube you want to use as the new manufacturers data sheets are out there (particularly JJ). If you can't find one you need write me back.

Good listening
Bruce

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