DIY Audio Projects Forum

Hi Everyone, Just FYI, but I am using the original KT88 Odd Blocks for sub woofer amps in the 50 HZ and down range. Excellent results on my rather efficient 7.5 cubic foot subs. The main amps are a pair of Oddwatt KT120 mono blocks. My concerns about the quality of sound from the sole source KT120s has so far been unfounded. You need to hear a set of the KT120 Oddwatts to believe them. I am still amazed about the low level response with distortion in the range that is normally reserved for preamps (about 0.06% at 1 watt rms). This is likely a good part of the reason that the amps have great inner detail in complex passages. Nice thing about the amps is the tubes are cheaper than most KT88s. Transformers sadly are not though. Also use 12SL7s as drivers like in the original project. They seem to have slightly better resolution over the 5751. More on that subject in a few days as I now have the new JJ 5751s to compare to the Sovtek and JAN Phillips ones. The JJ tube is quite a bit different in construction than the other two. It looks more like a dual RF triode than the long plate Sovtek and Phillips.

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Mate, I want to know what you find so keep me (us) informed.

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Projects: Oatley K272C - headphone amp in a tin can | "VoXDeuX" - Inexpensive two-way speaker | "VoXItBassO" - stereo 12" sub-woofers ]
Website: [ retro-thermionic ]

Hi, This is a copy of a post I made on another thread. If anyone has tried these transformers let us know how they worked out.


Hi Everyone, Thanks to another Diyer on a different thread, I checked out the following link.

http://www.vvttransformers.co.uk/output_pp_tx.htm

I have not tried these transformers and really can't comment on how they sound, but it seems they might be a good alternative to Edcor ones for our folks in Europe. I didn't price them out. They could be more than the Edcors including shipping. The suggested tube uses are not as I would list, so go by the wattage and primary impedances. It would probably be wise to verify that they can handle the idle current of the tubes as well. This is because many transformers are rated for use on class AB amplifiers that have lower idle currents. Class A amplifiers (all Oddwatts) have the highest DC current at idle and not at full power output like AB amplifiers.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Hi Everyone, A product / project update. I have found that UF4007 rectifiers seem to rectify better than the MUR ones that show in some of the schematics. This is also true of Poddwatt applications where they are used for the B+. As rectifiers in the DC heaters (if used) the MUR ones are fine. I discovered this when comparing a pair of nearly identical amplifiers, it took almost a 50% increase in size of the first two filter capacitors to achieve the same level of ripple on the B+. Both were quiet and there was no audible hum at the outputs, just one measured better. With the cost of UF4007s being only a few cents each in quantities, it makes sense to use them in place of the more costly MUR series.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Bruce, I have had my Oddwatt Monoblock Octal Amps for a while but am jus getting ready to begin construction. What solder do you reccomend? I want to start assembly this weekend, but will get solder locally or order it ASAP.

Hi, Many individuals like the lead free "silver" based solders. I find them prone to cold solder joints. I use 20 gauge (.040) 63/37 lead based. 0.032 size is also good. They melt at lower temperatures and makes a nice shiny bonds. Easy to tell when yo make a cold solder joint. I'm all for the environment, but I doubt the amount of solder used in building the amplifiers will matter. There is more lead in one automobile battery than I'm likely to use in a lifetime. Since you are in the Phoenix area as I recall, Frys Electronics will have the solder.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Silver takes a little more experience to work with than does the standard 60/40 stuff. It also requires considerably more heat. I do use silver alloy, but generally only where junction strength is paramount. Silver provides a structurally stronger bond. For GP work, I'll still use the 60/40. Unlike 60/40, silver alloy will not produce the nice shiny joint a 60/40 gives. You can tell a bad silver joint by looking for cracking/crazing in the joint, not by its lack of shiny.

On another note.....
I see Tube Depot is now selling the KT77 OddBlock.

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

Science without religion is lame, religion without science is blind. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board
Current project: 6V6 "pseudo Champ" prototype
Still to come (On hold): 6CG7/12AX7 Guitar amp (modified FireFly)

Hi, Tube Depot actually has been selling the amplifiers for about 18 months now.

On the silver solder, my biggest beef with it is the amount of heat needed. I don't like to subject components to more than is necessary.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

I have been trying to tune-up the KT88 - 6SL7GT Oddwatt during the last weeks...

With the signal generator and the scope I saw something really weird, so for guide-up, I used 1KHz sine-wave up to 2.0Vpp.

At the first instance it seems like the 6SL7GT has to much gain for the KT88, but let me explain...

I get a clean frequency response on the driver tubes, but only up to 1.8Vpp no matter what audible frequency I inject.

Beyond that value, there is distortion on the 6SL7GT output and of course on KT88´s BUT...

BUT if I raise-up the KT88´s current, say about 10mA per pair the distortion disapears everywhere.

I find this very strange. At the first instance I tought it could be due to a voltage drop on the PSU and so on the 6SL7´s but NO.

Could this due to bad impedance matching between the 2 stages? When I increase the KT88 current, its impedance lowers, right?

Conclusion:

- I believe that it is better to avoid distortion from the pre-amplifier stage, so that all distortion you get "should" come from the output stage.

- I am getting distortion before the KT88´s start to clip


Any advise or ideas are very welcome!


Cheers,

Miguel

Hi Miguel, Curious. I find that the gain of the 6SL7 is just about right as you need nearly 60 v pp to drive the KT88s. I suspect that there is something about the way you are measuring the signal that is giving strange results. If the amp sounds great then it has to be getting sufficient drive. I can get nearly +/- 70 volts from most 6SL7s before any noticeable wave form distortion. If you are measuring the input... then if you go to about 1.6-2.0 volts input you will exceed the voltage on the lower cathode and it will go non-linear. Just a function of how tubes work. But at about 1.4 volts input you will be at or slightly above the maximum designed class A output of the amp. Above that range you will run into distortion caused by the LM317 as it goes into the non-linear region when there is less than 4 volts across it. You get a really odd looking wave with a kink in it. If you increase the current, then the output level at which this occurs will be higher, but you are getting into a region where you can exceed tube dissipation. A question... is the variation of the amplifier one that has the small positive voltage applied to the KT88 grids? If not Let me know and I'll send you that variation. To overcome the 4 volt limit on the LM317s I add about 5-6 volts to the grids of the output tubes. This causes a like increase in the cathode voltage. (and will make the 317 dissipate a bit more - but still safe). With this modification you can drive the tubes a little harder (at the same original current) before distortion sets in. The difference in the KT88 version is between 2-3 watts. A level change that is not audible. It makes a big difference in the EL84 amps and is now"standard" in all those designs. A one watt increase there is significant as they only deliver 4.5 or so.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project