Hi Folks,
I am relatively green to the tube amp world but I have successfully completed one project and I am looking towards building my second power amplifier.
I am keen to build a PP amp with KT88 output tubes and I have already searched the board but I would greatly value some specific advice.
I would like to build a stereo amp, not two monoblocks. I am having difficulty finding a decent schematic and parts list for a PP stereo KT88. Can anyone help with a recommendation?
I would like something that has –
1. Tube rectification
2.50-60 w/channel output (I think that this should be very achievable with the KT88’s in PP configuration)
3.Relative availability of parts, esp. with respect to the output transformers
I will build my own chassis so this is not an issue.
I looked at the Dynaco MK III schematic and I like the look of this amp (a lot) but I am keen for a stereo amp from the outset and not two monoblocks (just a personal thing). I was toying with the idea of just building the two monoblocks into one chassis. My main concern with this option was the power transformer. I don’t want to use two – weight and size start to become a real issue. I have looked at the Sunn P782 S transformer (Rated 400-0-400V [800VCT] at 200 ma – maybe up to 300ma continuous if pushed) but this transformer is specifically for the MK III monoblock and it is not clear to me that it has enough grunt to use in a stereo build. I am not sure if there is another transformer around that might be suitable to use instead?
Does anyone have a solution? Either a transformer/power option for a 'stereo' version of the MK III (??MK IV) or another design altogether taking into account the requirements mentioned above.
Thanks in advance for the advice.
Rob
I am relatively green to the tube amp world but I have successfully completed one project and I am looking towards building my second power amplifier.
I am keen to build a PP amp with KT88 output tubes and I have already searched the board but I would greatly value some specific advice.
I would like to build a stereo amp, not two monoblocks. I am having difficulty finding a decent schematic and parts list for a PP stereo KT88. Can anyone help with a recommendation?
I would like something that has –
1. Tube rectification
2.50-60 w/channel output (I think that this should be very achievable with the KT88’s in PP configuration)
3.Relative availability of parts, esp. with respect to the output transformers
I will build my own chassis so this is not an issue.
I looked at the Dynaco MK III schematic and I like the look of this amp (a lot) but I am keen for a stereo amp from the outset and not two monoblocks (just a personal thing). I was toying with the idea of just building the two monoblocks into one chassis. My main concern with this option was the power transformer. I don’t want to use two – weight and size start to become a real issue. I have looked at the Sunn P782 S transformer (Rated 400-0-400V [800VCT] at 200 ma – maybe up to 300ma continuous if pushed) but this transformer is specifically for the MK III monoblock and it is not clear to me that it has enough grunt to use in a stereo build. I am not sure if there is another transformer around that might be suitable to use instead?
Does anyone have a solution? Either a transformer/power option for a 'stereo' version of the MK III (??MK IV) or another design altogether taking into account the requirements mentioned above.
Thanks in advance for the advice.
Rob
No reason in the world not to use the Dyna design in stereo; there are better designs out there, but at the cost of complication. The Dyna is still excellent.
Given the total mass of such an undertaking, I think that you're not going to notice whether you have one larger power transformer or two smaller ones. Doing a quick look-see at the Hammond range, the P-T278CX seems like it would work fine for two channels. For more $$$, the One Electron BFT-1B looks even better.
Given the total mass of such an undertaking, I think that you're not going to notice whether you have one larger power transformer or two smaller ones. Doing a quick look-see at the Hammond range, the P-T278CX seems like it would work fine for two channels. For more $$$, the One Electron BFT-1B looks even better.
Hi Sy,
The hammond P-T278CX is not going to work for me as it only has a 110v primary. The 240v equivalent does not seem to have a 5v winding. HOLY SMOKE, the One Electron BFT-1B looks like the mother of all transformers - I could really do some serious damage with that thing. Maybe two smaller ones is a better idea....however if you know where to buy them I might just have a quick look. Hmm.
The hammond P-T278CX is not going to work for me as it only has a 110v primary. The 240v equivalent does not seem to have a 5v winding. HOLY SMOKE, the One Electron BFT-1B looks like the mother of all transformers - I could really do some serious damage with that thing. Maybe two smaller ones is a better idea....however if you know where to buy them I might just have a quick look. Hmm.
Toroidal power transformers are considerably lighter for a given rating - worth looking into. Cost should be similar. Search eBay for ANTEK transformers for an example. I used a Plitron toroid in a small amp and wound my own heater winding on it... of course, it can't have a potted center in this case!
Here are some nice designs with the KT88,
this one uses an interstage transformer for the fase splitting:
http://www.diyparadiso.com/boully1.htm
A similar design:
http://www.audiodesignguide.com/my/pp3.html
Monoblocks but with tube rectifier:
http://www.diyparadiso.com/ppkt88mono1.htm
Another design:
http://wduk.worldomain.net/acatalog/AmpKits.html
this one uses an interstage transformer for the fase splitting:
http://www.diyparadiso.com/boully1.htm
A similar design:
http://www.audiodesignguide.com/my/pp3.html
Monoblocks but with tube rectifier:
http://www.diyparadiso.com/ppkt88mono1.htm
Another design:
http://wduk.worldomain.net/acatalog/AmpKits.html
The One Electron BFT-1B is probably your best bet for a cheap, one transformer stereo setup. It's $160 at tubesandmore.com in the US now.
If it were me though, I'd take that $ and buy two slightly smaller power transformers, hammond or edcor and make a dual mono amp on a single chassis. You'll get a better sound that way for almost the same price...
Take a look at this page, the 110 is nice and only $60
If it were me though, I'd take that $ and buy two slightly smaller power transformers, hammond or edcor and make a dual mono amp on a single chassis. You'll get a better sound that way for almost the same price...
Take a look at this page, the 110 is nice and only $60
Hey thanks Cycline3,
My difficulty with some of the transformers that i have seen (including the 110) is the 110v primary winding.
I appreciate the advice re: the quality of sound and the single transformer option. I think after reading the comments above I will go for the two transformer option as you suggest. The triode electronics guys have a 240V version design for the MKIII and that may be my best option.
Cheers,
Rob
My difficulty with some of the transformers that i have seen (including the 110) is the 110v primary winding.
I appreciate the advice re: the quality of sound and the single transformer option. I think after reading the comments above I will go for the two transformer option as you suggest. The triode electronics guys have a 240V version design for the MKIII and that may be my best option.
Cheers,
Rob
Rob11966 said:
Oh yeah.. sorry... sometimes I forget we aren't all in the states... the hammond 3 series has world primary support you could look there...
Here's a link to some..
You could always put two 110v primaries in series 😀 - actually, that might not be as daft as it sounds 
Seriously, the Amplimo toroid transformers are pretty good, I think, (all for 230v mains, being of Dutch manufacture), as long as you don't worry about the noise from the mains. Trouble is, they seem all to be intended for cap input filters, which you may not like.
I use an Amplimo 7N607 power transformer, which has 340v @700mA, 50v @100mA and 6.3v @6.8A windings. I'm using it to power an EL34 triode-strapped PP stereo amp. Amplimo does have a higher current model but it's quite a bit more expensive.
Seriously, the Amplimo toroid transformers are pretty good, I think, (all for 230v mains, being of Dutch manufacture), as long as you don't worry about the noise from the mains. Trouble is, they seem all to be intended for cap input filters, which you may not like.
I use an Amplimo 7N607 power transformer, which has 340v @700mA, 50v @100mA and 6.3v @6.8A windings. I'm using it to power an EL34 triode-strapped PP stereo amp. Amplimo does have a higher current model but it's quite a bit more expensive.
My experience in this field is rather limited, but I am using a Hammond 374BX 375-0-375, 6.3, 5.0 175mA transformer in my TubeLab SimpleSE. It does run warm - the laminations are quite hot to the touch but the covers are only warm. Apart from that, it is absolutely dead quiet and does not look too bad. The amp itself is very quiet too.
I have used Antique Electronic Supply http://www.tubesandmore.com/ for tubes, chokes, transformers caps etc and found them very good. I picked up the power transformer on a trip to the USA so I cannot tell you how much postage would be to Australia. I have had some other components sent by them to Australia and they happily provided a quote for the postage costs before committing to the order.
Hammond make a 400-0-400 465mA transformer, but it does not have a 5.0v tap, just 6.3v. Guess if you want tube rectification you will need two power transformers, in which case you may as well build monoblocks.
Cheers,
Chris
I have used Antique Electronic Supply http://www.tubesandmore.com/ for tubes, chokes, transformers caps etc and found them very good. I picked up the power transformer on a trip to the USA so I cannot tell you how much postage would be to Australia. I have had some other components sent by them to Australia and they happily provided a quote for the postage costs before committing to the order.
Hammond make a 400-0-400 465mA transformer, but it does not have a 5.0v tap, just 6.3v. Guess if you want tube rectification you will need two power transformers, in which case you may as well build monoblocks.
Cheers,
Chris
Hi Chris,
Thanks for the info. Let me tell you, the postage costs from the States are hideous. I airfreighted two output transformers and one power transformer recently and it cost me about USD 120 (from memory) - half the total cost of the transformers! The more I think about it the more I think that two transformers are the way to go. I am going to look into the toroidals first though.
Rob
Thanks for the info. Let me tell you, the postage costs from the States are hideous. I airfreighted two output transformers and one power transformer recently and it cost me about USD 120 (from memory) - half the total cost of the transformers! The more I think about it the more I think that two transformers are the way to go. I am going to look into the toroidals first though.
Rob
Toroids are very convenient, but they have a major disadvantage- they're too good. They will faithfully transfer every bit of noise and hash appearing on one winding to all the other windings. If you go that route, use separate transformers for the heaters so that noise from the B+ circuit isn't coupled back into the tubes via the heaters.
Rob11966 said:
In case to don't already know about them, an Australian distributer of just about everything you need to build a valve amplifier (including Hammond transformers) is EVATCO:
http://www.evatco.com.au/
Cheers,
Glen
Hi Glen,
I have looked at Evatco in the past. It seems to me that it would be cheaper for me to buy the Dynaclone transformers from the States and even with shipping the cost would be lower than the Hammonds from Evatco. I appreciate that we are not comparing apples with apples and I don't know how the quality of the Dynaclone compares with the Hammonds but I am very happy with the Dynaclone transformers in the ST 70 that I have just built.
Cheers,
Rob
I have looked at Evatco in the past. It seems to me that it would be cheaper for me to buy the Dynaclone transformers from the States and even with shipping the cost would be lower than the Hammonds from Evatco. I appreciate that we are not comparing apples with apples and I don't know how the quality of the Dynaclone compares with the Hammonds but I am very happy with the Dynaclone transformers in the ST 70 that I have just built.
Cheers,
Rob
Yes I am aware of evatco. As much as I am happy to help out a local distributor, last time I looked their prices were not even close. For instance, Hammond 12*10*2 chassis $40.15 AUD (around $33 US), $19.50 US at Antique. Hammond 374BX transformer $254.50 AUD (around $211 US), Antique $92.35 US. Hammond choke 193H $63.25 AUD, ($52 US)at evatco, Antique $26 US. Tubes are around twice the price of Antique as well. You get the picture. Even with the freight from the USA to Australia being about the same as the transformer price you would still be ahead. Maybe evatco has been stung with the recent improvement of the AUD vs USD. I have not contacted them to see if they are flexible with price, has been easier to just order from the companies I trust.
There is a new option for overseas mail from the U.S. - International Priority Mail is $37 for a flat-rate box about the size of a shoebox, up to 40 pounds.
Rob, also not sure what price point you are aiming for with this next project, but I can recommend the service from Edcor transformers. They deal via email with payment via paypal. They will email you with a quote for the cost of freight (USpriority airmail from memory). TubeLab did a comparison of various transformers on his site here http://www.tubelab.com/Output_Transformers.htm Note that he is comparing single ended transformers here, but it does give an indication of the quality of the Edcor transformers that are quite a bit cheaper than the Hammonds. Here is a link to the enclosed push pull transformers http://www.edcorusa.com/products/transformers/cxpp/index.html $60-$80 each for the larger ones...
Look forward to seeing this project progress. My latest has stalled, very weird oscillations and no time to fix due university studies at the moment!
Good luck!
Look forward to seeing this project progress. My latest has stalled, very weird oscillations and no time to fix due university studies at the moment!
Good luck!
Time to display ignorance!!
Hey, thanks for the tips folks.
The Edcor transformers look interesting. The quote for transformer shipping turned out to be USD 190. Armed with the above information concerning the new USPS special rate, I have asked for a re-quote. Will let you know how it goes.
Now is the time for me to air my ignorance. I can build things well if given the proper instructions but my knowledge of electronics is still on the steep part of the exponential curve. I have been looking at the MK III circuit diagram, and I am struggling with a couple of differences when compared with the Dynaco 70 schematic that I used for my first amp.
Firstly, I cannot find a parts list anywhere with specs, so if anyone has one or can point me in the right direction, I would be grateful.
Secondly, it appears to me that all of the caps (not just the PSU ones) are polarized (as indicated by the bent line on one side, which I believed was the symbol for a polarised cap), as compared with the Dynaco -70. Is this the case or am I just mis-interpreting the schematic? I am thinking that only the ones with the '+' signs next to them are supposed to be polarized?
Thirdly, in the absence of a parts list, is it correct to assume that resistors are 0.5 W and capacitor values are uf unless otherwise specified?
Finally, is it the case that the capacitors labeled 20/525 are 20uf, 525v rated?
Thanks for the help again and sorry for the stupid questions. Not having had a formal electronics education, it is little things like this that trip me up. The building is the easy part!
Rob
Hey, thanks for the tips folks.
The Edcor transformers look interesting. The quote for transformer shipping turned out to be USD 190. Armed with the above information concerning the new USPS special rate, I have asked for a re-quote. Will let you know how it goes.
Now is the time for me to air my ignorance. I can build things well if given the proper instructions but my knowledge of electronics is still on the steep part of the exponential curve. I have been looking at the MK III circuit diagram, and I am struggling with a couple of differences when compared with the Dynaco 70 schematic that I used for my first amp.
Firstly, I cannot find a parts list anywhere with specs, so if anyone has one or can point me in the right direction, I would be grateful.
Secondly, it appears to me that all of the caps (not just the PSU ones) are polarized (as indicated by the bent line on one side, which I believed was the symbol for a polarised cap), as compared with the Dynaco -70. Is this the case or am I just mis-interpreting the schematic? I am thinking that only the ones with the '+' signs next to them are supposed to be polarized?
Thirdly, in the absence of a parts list, is it correct to assume that resistors are 0.5 W and capacitor values are uf unless otherwise specified?
Finally, is it the case that the capacitors labeled 20/525 are 20uf, 525v rated?
Thanks for the help again and sorry for the stupid questions. Not having had a formal electronics education, it is little things like this that trip me up. The building is the easy part!
Rob
Attachments
- Status
- Not open for further replies.