Found a thread started by Tubemack back in 2009.
http://http://www.diyaudio.com/forums/tubes-valves/156699-mullard-5-20-kt88-pp-blocks.html
His goals were very much in-line with what I want to do and looks like he decided to go with a modified Mullard schematic. Will take me some time to read through 100+ pages of posts, but it looks like there were many issues with the build. Not sure if he ever finished. Reinforces my instincts to stick with a proven schematic.
On the plus side, Eli had recommended Antek torroid power with Edcor output transformers based on the schema so I think I'm in the right ball park.
http://http://www.diyaudio.com/forums/tubes-valves/156699-mullard-5-20-kt88-pp-blocks.html
His goals were very much in-line with what I want to do and looks like he decided to go with a modified Mullard schematic. Will take me some time to read through 100+ pages of posts, but it looks like there were many issues with the build. Not sure if he ever finished. Reinforces my instincts to stick with a proven schematic.
On the plus side, Eli had recommended Antek torroid power with Edcor output transformers based on the schema so I think I'm in the right ball park.
Okay well one push pull amps on you build options list was Pete's Engineers amp which puts out 18 watts. You could easily get 15 watts from a KT88 single ended amp. The difference between 15 watts and 18 watts is not much.
What speakers do you have?
What is the size of the room?
What music do you like and how loud do you like it?
I didn't catch the power output which you require, or the output transformers which you have.
4k3 p-p w UL taps run Class A KT88 for 25W.
gain stage to cathodyne splitter, RC coupled to the output tubes.
Common pot in the OP cathode cct wiper to cathode bias w/wo bypass, NFB?
Wiper to CCS and you'd start be cookin' good, but ... do you need 25W?.
This aint my thread or interest, I'm buggin' out... others are in a better place to help - my maps are all in my head with this (bet they work a treat though).
Cheers,
Hanze.
4k3 p-p w UL taps run Class A KT88 for 25W.
gain stage to cathodyne splitter, RC coupled to the output tubes.
Common pot in the OP cathode cct wiper to cathode bias w/wo bypass, NFB?
Wiper to CCS and you'd start be cookin' good, but ... do you need 25W?.
This aint my thread or interest, I'm buggin' out... others are in a better place to help - my maps are all in my head with this (bet they work a treat though).
Cheers,
Hanze.
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Similar to many first time DIY folks, I'm looking for monoblocks in the neighborhood of about 60 watts. This appears to be about what can be expected from a good 2 x KT88 design. Initially, I'm planning to use the amps with relatively inefficient DIY 2-way design built with Dynaudio drivers. I'm guessing they'll be about 86db. Beyond that, who knows what I might end up buying or making for speakers, but it will be nice to know my amps will be able to drive most sets.
After quickly reading though 100 pages of the TubeMack thread, it seems only two members completed their builds and posted their finals. Although they started with a Mullard design, it apparently kept evolving to the point it was not long a "Mullard", and the finished result was still somewhat problematic.
So, I'm still searching for the right design. The list still stands:
Also, I want to reiterate how I like Eli's ideas about using an Antek torroid for power and some Edcor trafo for outputs. He makes a very good argument for their use from a cost/benefit standpoint.
After quickly reading though 100 pages of the TubeMack thread, it seems only two members completed their builds and posted their finals. Although they started with a Mullard design, it apparently kept evolving to the point it was not long a "Mullard", and the finished result was still somewhat problematic.
So, I'm still searching for the right design. The list still stands:
- MkIII
- M-125
- Engineer's amp
- Some Mullard style topology
Also, I want to reiterate how I like Eli's ideas about using an Antek torroid for power and some Edcor trafo for outputs. He makes a very good argument for their use from a cost/benefit standpoint.
Eli's recommendation is spot on.
You can't go wrong with Antek power iron, they are very high quality. Only downside is the lack of a 5 volt winding for use with the more common vacuum rectifiers, which isn't a big deal, as solid state diodes make more sense at these power levels anyway
Some of the snobby types don't care for Edcor output transformers, but let them waste their money on overpriced parts. Edcor makes good stuff. A good recommendation is to overspec your transformers for magnetic headroom. It will usually make them play better with feedback loops, and they will go lower frequency before hitting any distortion.
The engineers amplifier is a great option, and you can always wire octal sockets from the chassis to the board and run KT88 instead of the compactrons he used. I would still recommend using the pentode front end of the design though. It would be pretty easy to change a few values to make it work at higher voltages that the KT88 will want to see.
You can't go wrong with Antek power iron, they are very high quality. Only downside is the lack of a 5 volt winding for use with the more common vacuum rectifiers, which isn't a big deal, as solid state diodes make more sense at these power levels anyway
Some of the snobby types don't care for Edcor output transformers, but let them waste their money on overpriced parts. Edcor makes good stuff. A good recommendation is to overspec your transformers for magnetic headroom. It will usually make them play better with feedback loops, and they will go lower frequency before hitting any distortion.
The engineers amplifier is a great option, and you can always wire octal sockets from the chassis to the board and run KT88 instead of the compactrons he used. I would still recommend using the pentode front end of the design though. It would be pretty easy to change a few values to make it work at higher voltages that the KT88 will want to see.
I did this around 18 months ago - did my first build as a KT88 PP based on Williamson topology, using 12AX7 and 12AU7s for the gain stages, Hammond transformers and a custom made mains transformer. It was a definite learning curve but I was pleased with the result and it's my Daily Driver these days.
Since then I've continued building other projects, and if I had to build this one again, there's lots I would do differently. But, I am still happy with it.
Good luck with your build
Since then I've continued building other projects, and if I had to build this one again, there's lots I would do differently. But, I am still happy with it.
Good luck with your build
The Universal Driver Board is not a new concept. It's actually about 10 years old and started here:
6L6GC AB2 Amp
The driver circuit was already designed as that thread unfolded, and was hammered into a proven circuit during that long thread. It's a good place to learn quite a bit if you have the time to read it all.
True, the UDB has been hibernating on a shelf for quite some time, and I'm committed to building a KT88 push pull amp out of it first since several people are going down that path simultaneously. I'm not saying whether it's for you, nor not, but you might learn a bit about KT88 based push pull amps by reading along in the UDB thread too. Progress will be in spurts as life keeps putting things on hold periodically.
Now, that's more of a new concept. I think there has been a total of 1 1/2 built. I made a guitar amp using a DIY guitar preamp design with a phase splitter to drive an SSE board with each channel doing one phase, and feeding a single push pull OPT. It rocked and made over 50 watts from a pair of 6L6GC's. The other one was Anchan's stereo.
I never had a MarkIII, so I can't say for certain......I have however built three of Pete's original Engineers amps. Being an electrical engineer myself, I bought a board while making the statement that I would be seriously disappointed if I didn't get at least 50 WPC out of it.
I made a gross underestimate. I had 50 WPC flowing out of that thing 5 minutes after I got it built. Within a few weeks I saw 250 WPC with minimal modifications. I published a 125 WPC flavor of that board which required a few minor modifications and some parts changes. I built 2 of them and at least 10 more were build by other readers in that thread.
That design was the basis of Pete's more conservative 50 watt monoblock. Sound? It has a very dynamic sound. Loud rock, big bass no problem (with appropriate OPT's). It still does a good job on more detailed music too. Reliability? Well I had a habit of cranking mine up to full power, opening the windows and going outside and mowing the lawn without hearing the lawnmower......125 WPC through 96 dB speakers will do that. It also ate my guitar playing without blowing up. Again, that long thread is a good learning opportunity for amp mods, tweaking, and TV sweep tubes.
My 125WPC version of Pete's board AND my KT88 version of the UDB amp both used the same OPT's...Edcor CXPP100-MS-3.3K.
Both could be built with Antek power toroids, but it would take one or two Anteks plus a small isolation transformer to get into the 60 WPC+ range.
All amps that run TV sweep tubes at power levels beyond 20 watts or so must run them in pentode mode due to their low screen voltage requirements (150 to 175 volts). Power output in the 60+ watt range requires around 500 volts or more. These different voltages are easily handled with a custom transformer, or two standard Anteks.
Thanks again for all the comments. It's really helpful hearing from others with hands-on experience!
Some more reading other threads and almost started to lose focus again. The 50w Engineer's Amp is starting to rise to the top of my list. Could it be the clouds in my head are starting to disperse?
Wiring sockets sounds doable. When you say changing values, I assume you're referring to the values of the caps and resistors? Definitely would like to run KT88s.
So, with the Antek I'd need to use either SS diodes or add another 5V source for use with either the Engineer's Amp, MkIII, or yet-to-be Tubelab UDB? Another extremely green question, but... might the Antek 4T360 be an option? Website shows it having two 360v/0.55A as well as two 6.3V/4A outputs?
Some more reading other threads and almost started to lose focus again. The 50w Engineer's Amp is starting to rise to the top of my list. Could it be the clouds in my head are starting to disperse?
Wiring sockets sounds doable. When you say changing values, I assume you're referring to the values of the caps and resistors? Definitely would like to run KT88s.
So, with the Antek I'd need to use either SS diodes or add another 5V source for use with either the Engineer's Amp, MkIII, or yet-to-be Tubelab UDB? Another extremely green question, but... might the Antek 4T360 be an option? Website shows it having two 360v/0.55A as well as two 6.3V/4A outputs?
How do you suppose a UDB with KT88 might sound? I recall reading somewhere you thought your 300 Beasts were the best sounding things you ever made. Dynamic is good, but I'm rather hoping to get as close as possible to the warmth and sweetness of the 300B SE designs. Yes, it's very subjective, but do you think a KT88/UDB could possibly be a better option for me than the Engineer's Amp?
I'm going to inject my opinion here.
It sounds like you've chosen the KT88 as your tube of choice. I don't think you can go wrong choosing anything George has had his hands in.
On the other hand, I know and trust Pete's designs, so if you can find a way to implement the KT88's into his board, then I think that would be a viable option for you.
Good luck in what ever you choose and please document your build for the rest of us
The Engineer's Amp was designed specifically for TV sweep tubes. All "real" TV sweep tubes run the screen grid at 150 to 200 volts. A KT88 wants 300 to 500 volts on its screen grid, and will only make a few watts with 150 volts on G2. In order to run a KT88 (or an EL34 or 6L6 type) in the Engineer's Amp, there are four choices. Run the KT88 in triode mode, run it in UL mode, run the screen grid directly from the B+ supply, or add a screen voltage regulator.
Triode mode will create the lowest distortion, have the best speaker damping factor, but produce lower power output than UL or pentode mode.
UL (ultralinear) mode is a compromise between triode and pentode modes, offering some of the advantages and disadvantages of each. A little more power for a bit more distortion and less damping. Some people love it, some hate it, I find it pleasing with some tubes, blah with others, and without merit at all for some tubes. The 307A DH pentode comes to mind.
The screen can be run from B+ in cases where the B+ voltage is below the maximum spec when the amp is at high power. This is probably the case here.
A screen grid regulator can hold the screen grid voltage constant under all operating conditions, which can improve the amp's ability to handle transients, and offer lower distortion at high power. If the screen is regulated, it is recommended that the negative bias be regulated as well, to avoid tube current change with line voltage change. This can become complicated, but is the best case for high powered amps.
I have run KT88's and 6L6GC's in Pete's original Engineer's Amp in triode mode. It's a simple socket wiring job. The screen grid circuitry on the EA board doesn't get connected to the octal socket.
I built the 300Beast maybe 12 to 15 years ago. It was one of a kind. At that time I had been making SE amps, some with 300B's. The few P-P amps I made used 6L6GC's or 1990's vintage Chinese KT88's which didn't work very well. I made the 300Beast and it had a dynamic sound that impressed me. It was created entirely from junk box parts and leftovers, including a pair of $16 OPT's from my guitar amp building projects. The circuit was a hodge podge of the trendy circuits of the day. The 300Beast refused all attempts to upgrade it, including some $$$$ OPT's. All upgrade experiments made it sound worse, and were subsequently reversed. It's power supply used photoflash grade electrolytics (junk). The amp blew a fuse during a Florida thunderstorm one day (not uncommon on any tube amp). I didn't have the right fuse so I stuck something bigger in thinking it was a typical power surge. I was however met with smoke and the stink of electrolytic goo. I believe that the power supply is the only dead circuit, but the amp has been sitting around ever since. Was it "better" than the stuff I make today? I don't know since no direct comparisons were ever made. I will attempt to power up it's remains with external power supplies before it meats the salvage operation........
The 300Beast and any push pull amp will give up some of the "300B SE" sound simply by virtue of the push pull amps partial cancellation of second harmonics. A push pull amp is capable of faster response to transient signals, and this is dependent on the peak current capability of the output tubes and power supply. KT88's have a higher peak current capability than 300B's and TV sweep tubes have a higher peak current than KT88's.
So the real comparisons should be a SE 300B amp VS the 300 Beast. Done that one, the Beast excels in the dynamics department, and gives up some of that SE "sweetness." For me, at the time, it was a welcome change. For the last few years I was in Florida, I did have several amps to choose from, just by turning a switch. When the 300Beast died, it was replaced by a modified SPP amp, which was replaced by a modified Engineers Amp.
The next obvious comparison would be the 300Beast VS a triode wired UDB design. That direct comparison never happened, since the two amps never existed at the same time. Why triode wired? If one were to compare something to a 300B, that something should at least be a triode. Back in the 6L6GC in AB2 thread I connected up a bunch of different tubes to the predecessor of the UDB. After listening to a bunch of tubes, I had decided that I would build myself a triode wired KT88 amp.....until I tried some 7403's (a military radar pulse tube). I recently spent two days playing with 7403's and the UDB, and have decided to cross the 7403's off the list for now. They have widly scattered parameters, so some serious testing, matching, and buen in will be needed before using these tubes. At the same time I tested some triode wired 307A tubes. These are directly heated pentodes intended for WWII radio transmitters. I have a bunch of them and they work well as substitutes for a 300B in my TSE 300B SE amps. I have a TSE with 307A's and like it.
I may try to fire up the old 300Beast with a bench power supply and compare it to the UDB with KT88's. I also plan to test the UDB design with 300B and 307A tubes.
My plan is to build several amps using parts that I already have on hand since the budget is essentially zero right now. The UDB with KT88's is being built currently. Breadboard testing is done and the chassis test fit and mockup for the final amp is on the bench.
What's next? I will build several amps using TV sweep tubes in power levels from 30WPC to 500 WPC......I have been itching to build a BIG amp for years, and have collected most of the parts. Those will wait though. The 300Beast has been sleeping for too long. I have several 300B tubes (Russian and Chinese, no expensive stuff) and about 20 307A's, a bunch of 45's and some 2A3's. Something will be built with the best sounding of those tubes. After that??????
I know how it sounds since I had one playing on the bench for a few weeks until I stole the OPT's for the chassis mock up. How do I convey this to someone else who has different everything from me? I don't know. A Youtube video recorded with a cell phone certainly will not work.....but I made one anyway.
YouTube
Note that the KT88 and the 6550 are NOT identical, however some manufacturers may stuff the same guts into each bottle. I have a EH 6550 paired with an EH KT88 in one channel of my amp. They match perfectly and I'm convinced that they are the same tube.
The difference is the topology of the driver section. The EA uses a pentode LTP to directly drive a TV sweep tube. This combination works well.
The UDB uses two LTP's made with triodes to perform the driver duties. It also has a mosfet follower buffer stage between the last LTP and the output tubes. TV sweep tubes do not really need the buffer since they do not need or want their grids driven positive. KT88's can benefit from a buffer, especially if driven hard. Tubes like the 300B really like a mosfet buffer as hundreds of TSE owners have realized.
I would suspect that the choice of OPT's and power supply may make a bigger impact on the sound than the choice of driver unless you are planning on running it at the edge of clipping often, then the mosfet buffer in the UDB will offer some advantage.
Note: you haven't stated how much power you want from this amp. Pete's monoblock EA is rated for 50 watts, so I assumed that's sufficient. The old 300Beast made 28 watts. I would suspect that a 300B based UDB amp could make a bit more, but I won't know that until I test it. If you are not in a big hurry, watch the UDB thread for a while to see how my amp, and a few other's turn out. Maybe "Son of a Beast" is more like what you want.
I will....even the mistakes.
Pictures of KT88 chassis mock up.
Attachments
Wow, thanks for the detailed response. Honestly, I think 50w would be adequate but all things being equal, I’d favor building even more power if possible and running it at less than full tilt.
I think you have a good idea of what I’m looking for. Generally speaking, the UDB KT88 mock-up you posted above checks off most of my wish list: torroid, Edcor OPT, and good amounts of power. As you said, it’s hard to tell the sonic character from the video but it was great watching anyways. I just fear chasing more power will get me an amp that sounds more SS than tube. Hmmm.... Son of a Beast vs the KT88? Both interesting options. Will keep reading your UDB thread to see what else I’m missing.
I think you have a good idea of what I’m looking for. Generally speaking, the UDB KT88 mock-up you posted above checks off most of my wish list: torroid, Edcor OPT, and good amounts of power. As you said, it’s hard to tell the sonic character from the video but it was great watching anyways. I just fear chasing more power will get me an amp that sounds more SS than tube. Hmmm.... Son of a Beast vs the KT88? Both interesting options. Will keep reading your UDB thread to see what else I’m missing.
If you haven't ever built anything from scratch, then I would recommend starting with a kit, especially one that provides some kind of technical support if things go wrong.
A preamp will be less expensive to start with, involve lower voltages, and will be a good spring board from which to move on to other builds.
A preamp will be less expensive to start with, involve lower voltages, and will be a good spring board from which to move on to other builds.
Been mulling over what I've learned so far. Lots of great advice.
George (I hope you don't mind me addressing you by name), I noticed there are 5 trimmers/pots on each UDB. Can you describe how they are used when setting bias? For example, would you begin by adjusting 2 pots for the input tubes, then 2 pots for output tubes, and I have no idea about the 5th? Is it common to have five pots or is this unique to the UDB?
Are you still planning on a "cookbook" KT88 design?
George (I hope you don't mind me addressing you by name), I noticed there are 5 trimmers/pots on each UDB. Can you describe how they are used when setting bias? For example, would you begin by adjusting 2 pots for the input tubes, then 2 pots for output tubes, and I have no idea about the 5th? Is it common to have five pots or is this unique to the UDB?
Are you still planning on a "cookbook" KT88 design?
The need for 5 pots is due to the "universal" nature. The two pots near the long row of connectors are the bias pots for the output tubes. They set the idle current through each tube.
Two other pots set the current through each driver tube (CCS in tail of LTP). A tube like a 12AX7 wants 1 to 2 mA, while a tube like a 6CG7 wants 10 mA.
The 5th pot sets the balance. It compensates for imperfect tubes, and is adjusted for equal plate voltage on the last driver tube.
There will be a detailed set of adjustment procedures in the other thread once it gets further along.
Two other pots set the current through each driver tube (CCS in tail of LTP). A tube like a 12AX7 wants 1 to 2 mA, while a tube like a 6CG7 wants 10 mA.
The 5th pot sets the balance. It compensates for imperfect tubes, and is adjusted for equal plate voltage on the last driver tube.
There will be a detailed set of adjustment procedures in the other thread once it gets further along.
Just another opinion here.
I built a KT88 SE power amplifier, which sounds great. Even better than an EL84 PP I built, which already sounds fantastic.
Very simple desing. Just a 12AX7 for gain, and a cathode follower driver. As simple as it gets; no active load or fancy circuits. Great as a first project, if you like to understand everything going on in your circuit.
Uses 10dB of global negative feedback and fixed bias.
With an OPT of 2k5 you get around 10W.
At first I thought that I was going to miss having much more power.
My previous solid state amplifier was 50W class A....
But I rarely go beyond 4-5 W, so I have some room. And more important, I enjoy all kind of music styles, and all people that have hear it love it. Even at max power.
In fact, I finally use it triode strapped, which sounds definitely better than ultralinear. Just a little bit less volume.
It depends on the listening room size.
And, of course that depends also on how inneficient your speakers are.
My speakers are 85 dB (2,83V / 1 m).
I built a KT88 SE power amplifier, which sounds great. Even better than an EL84 PP I built, which already sounds fantastic.
Very simple desing. Just a 12AX7 for gain, and a cathode follower driver. As simple as it gets; no active load or fancy circuits. Great as a first project, if you like to understand everything going on in your circuit.
Uses 10dB of global negative feedback and fixed bias.
With an OPT of 2k5 you get around 10W.
At first I thought that I was going to miss having much more power.
My previous solid state amplifier was 50W class A....
But I rarely go beyond 4-5 W, so I have some room. And more important, I enjoy all kind of music styles, and all people that have hear it love it. Even at max power.
In fact, I finally use it triode strapped, which sounds definitely better than ultralinear. Just a little bit less volume.
It depends on the listening room size.
And, of course that depends also on how inneficient your speakers are.
My speakers are 85 dB (2,83V / 1 m).
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