https://fr.aliexpress.com/item/3287...=google&aff_short_key=UneMJZVf&gclsrc=aw.ds&& albagn=888888&&ds_e_adid= &ds_e_matchtype=&ds_e_device=m&ds_e_network=x&ds_e_product_group_id=&ds_e_product_id=fr32870 580340&ds_e_product_merchant_id=107692202&ds_e_product_country=FR&ds_e_product_language=fr&ds_e_product_channel=online&ds_e_product_store_id=&ds_url_v=2&albcp=19000710609&albag=&isSmbAutoCall=false&needSmbHouyi=false&gad_source=1&gclid=CjwKCAjwxNW2BhAkEiwA24Cm9Kh6xMlMQD4yOnTYSSkxwrCab2Lg6_A7T-q0aNrdXSlnrvKVN-g2oRoCYFUQAvD_BwE
https://fr.aliexpress.com/item/1005...tform=google&aff_short_key=UneMJZVf&gclsrc=aw .ds&&albagn=888888&&ds_e_adid= &ds_e_matchtype=&ds_e_device=m&ds_e_network=x&ds_e_product_group_id=&ds_e_product_id=fr10050 02459261289&ds_e_product_merchant_id=105402784&ds_e_product_country=FR&ds_e_product_language=fr&ds_e_product_channel=online&ds_e_product_store_id=&ds_url_v=2&albcp=19000710609&albag=&isSmbAutoCall=false&needSmbHouyi=false&gad_source=1&gclid=CjwKCAjwxNW2BhAkEiwA24Cm9NvHBtrcIZmOZvSTCCK2UFLdmw0j76TcqVFp23_nbViqYo6HtASKHBoC4PoQAvD_BwE
Member THD+N has provided an example of an Eico HF-87 clone using Hammond output transformers. It is well-tested and stable. The Eico was a beautiful 35 wpc. I had one years ago and regret ever selling it. This would be a fun project to build and yield good power and great sound with readily available parts:
https://audiokarma.org/forums/index...dified-eico-hf-87.847158/page-6#post-15999739
https://audiokarma.org/forums/index...dified-eico-hf-87.847158/page-6#post-15999739
For the Dyna Mk III Circuit, be sure to purchase very well matched KT88 tubes.
Warning: Fixed Adjustable Bias 6550 tubes need Rg return resistors of 50k Ohms Maximum; the Mark III uses 100k Ohms, way to much (can not just change to 50k, the driver will distort to drive 50k.
If the tubes are not matched, the output transformer will hate you, and you will hate the performance of the output transformer too . . .
Saturated Laminations.
Global negative feedback never fixes saturation, it only makes it worse!
A small signal tube, like an input or driver tube that costs $8 is a bargain.
Just try and purchase a new CD for $8; then check the prices on a new Vinyl 33 LP.
You can not even purchase an $8 Quality Oil Filter for your car.
Put things in perspective, we are no longer living in the 1960s.
Warning: Fixed Adjustable Bias 6550 tubes need Rg return resistors of 50k Ohms Maximum; the Mark III uses 100k Ohms, way to much (can not just change to 50k, the driver will distort to drive 50k.
If the tubes are not matched, the output transformer will hate you, and you will hate the performance of the output transformer too . . .
Saturated Laminations.
Global negative feedback never fixes saturation, it only makes it worse!
A small signal tube, like an input or driver tube that costs $8 is a bargain.
Just try and purchase a new CD for $8; then check the prices on a new Vinyl 33 LP.
You can not even purchase an $8 Quality Oil Filter for your car.
Put things in perspective, we are no longer living in the 1960s.
There are many good amplifier candidates to choose from.
But if I read the original post correctly, the goals are:
Simplicity
Push pull
A teaching moment, but not enough to graduate a 4 year engineer.
Not break the bank
Moderate power
I think some posts may be deviating From the above criteria, and To some favorite amplifiers.
But if I read the original post correctly, the goals are:
Simplicity
Push pull
A teaching moment, but not enough to graduate a 4 year engineer.
Not break the bank
Moderate power
I think some posts may be deviating From the above criteria, and To some favorite amplifiers.
It would help a lot to know more about the PT's you can use. Did you want 30-40W/channel, or total power? Need some specs for your PS options to get much further.
Good summary except for the push pull requirement. I just assumed that you have to go PP to get the power. I also value performance like everyone else. An exercise like this takes a lot of time and is relatively expensive so I want it to be an amp that I like as well..... a tall order!
Point to point wiring, at least as much as possible is always better for learning IMO. I like the simplicity of using circuit boards now but I would not have learned as much if I had started with shake and bake kits.
As far as tubes, I'm very open. I found 8 Hammond EL84s last night that I pulled from an amp years ago but I'm not married to using them. I have a large stash of tubes of many varieties and I don't mind buying tubes as well.
I must say I've been spoiled building simple, low wattage DHT amps and later, First Watt clones over the years, but I use JBL/Altec drivers which mate well with these designs. My favorite amp is a 4 chassis GM70 amp today but it was a very ambitious build that took me a couple years to finish. I also rebuilt a Dynaco ST-70 a couple years ago but sold it.
Thanks for all the input! I will pour over the suggestions next couple days. I do like the idea of the Dynaco Mark III amp and the Eico clone Grover mentioned.
Over the grand service life of renown makes; however, the GZ34 or other rectifiers is often the first casualty of wear in an amp. The tube with the longest service tube in one of my amps is the 807´s which is soon 65 years, run nearly every weekend. When it comes to their replacement, the drawback is the UX5 base and top cap., but as we all know the 807 was originally a transmitting tube. So much for originality. The next is as you rightly mentioned, Mullard EL34´s at the same as yours.
Bench Baron
30-40 watts/channel.
I can model/build any power supply needed, solid state or tube rectified so this is not a concern.
I have to buy PTs. All mine are SE.
Do you know if the 12SL7 (V1) is shared by both channels or if he uses one per channel?
Recently, the HK Citation II has been successfully cloned using some cheap Antek OTs:
https://www.diyaudio.com/community/threads/citation-2024.416813/#post-7778673
Usually this Amplifier gets hyped up so much that DIY is unthinkable without possessing the original HK OTs. The reality is that the "local" N Fdbks ( here just meaning: NOT including the OT in the loop ) reduce the loop gain for the global Fdbk thru the OT to similar levels as a typical Mullard 5-20 clone/variant/upgrade. ( EICO HF89 ) This HK amplifier with KT88s meets the power requirements stated.
In any case, the same OTs could build the EICO Amp as well.
The HK Cit II could be even further modded to become a -Low Cost- amplifier. The $12 12BY7s can be replaced by plentiful $3 6197 tubes almost transparently. ( just a pin-out difference, same gm, mu factor and Rp ) The $240 to $300 a Quad of KT88s could be replaced by $3 6HD5 tubes IF the UL mode is dropped. ( Stew Hegeman himself said the UL mode was a mistake for this amplifier )
As for all the mysterious local FDBK loops, they can mostly stay the same. The inner most loop around the 12BY7/6197 drivers just lowers the driver output Z, and the crossed KT88 plate to 12BY7/6197 driver grid loops perform the "local" N FDBK for the KT88s or 6HD5s to linearise them. Without the UL mode, the plate to driver FDBK resistors -could- be dropped in value slightly to make up for the missing UL loops.
----------------------------
Another approach would be to upgrade ( for more power ) the highly respected RCA SP-10 amplifier. This amplifier uses a -conceptually- similar design as the HK Cit II but with some detail differences. Similar tubes and OTs could be used, 12BY7/6197 and KT88/6HD5 tubes should work fine, but many other combos could work as well. The RCA uses triodes in the front end, and pentode drivers. The CIT II uses a 12BY7 front end. Both use pentode drivers. The HK reduces the driver gains with a local loop to be similar to triodes for Zout. The main driver difference being non-crossed FDBKs to the SP-10 driver cathodes instead of crossed FDBKs to the Cit II driver grids. Similar results.
The RCA SP-10 needs an overdue upgrade for today's power needs, a very useful exercise.
Of course, either of these schemes can have the N FDBKs reduced ( higher value FDBK resistors ) if you want less precision and more traditional tube sound.
Citation II:
RCA SP-10:
https://www.diyaudio.com/community/threads/citation-2024.416813/#post-7778673
Usually this Amplifier gets hyped up so much that DIY is unthinkable without possessing the original HK OTs. The reality is that the "local" N Fdbks ( here just meaning: NOT including the OT in the loop ) reduce the loop gain for the global Fdbk thru the OT to similar levels as a typical Mullard 5-20 clone/variant/upgrade. ( EICO HF89 ) This HK amplifier with KT88s meets the power requirements stated.
In any case, the same OTs could build the EICO Amp as well.
The HK Cit II could be even further modded to become a -Low Cost- amplifier. The $12 12BY7s can be replaced by plentiful $3 6197 tubes almost transparently. ( just a pin-out difference, same gm, mu factor and Rp ) The $240 to $300 a Quad of KT88s could be replaced by $3 6HD5 tubes IF the UL mode is dropped. ( Stew Hegeman himself said the UL mode was a mistake for this amplifier )
As for all the mysterious local FDBK loops, they can mostly stay the same. The inner most loop around the 12BY7/6197 drivers just lowers the driver output Z, and the crossed KT88 plate to 12BY7/6197 driver grid loops perform the "local" N FDBK for the KT88s or 6HD5s to linearise them. Without the UL mode, the plate to driver FDBK resistors -could- be dropped in value slightly to make up for the missing UL loops.
----------------------------
Another approach would be to upgrade ( for more power ) the highly respected RCA SP-10 amplifier. This amplifier uses a -conceptually- similar design as the HK Cit II but with some detail differences. Similar tubes and OTs could be used, 12BY7/6197 and KT88/6HD5 tubes should work fine, but many other combos could work as well. The RCA uses triodes in the front end, and pentode drivers. The CIT II uses a 12BY7 front end. Both use pentode drivers. The HK reduces the driver gains with a local loop to be similar to triodes for Zout. The main driver difference being non-crossed FDBKs to the SP-10 driver cathodes instead of crossed FDBKs to the Cit II driver grids. Similar results.
The RCA SP-10 needs an overdue upgrade for today's power needs, a very useful exercise.
Of course, either of these schemes can have the N FDBKs reduced ( higher value FDBK resistors ) if you want less precision and more traditional tube sound.
Citation II:
RCA SP-10:
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I should add that the RCA SP-10 does NOT use a global FDBK loop, using instead higher local loop gain (full pentode) in the driver stage. You could almost make the OT there with baling wire. Maybe appealing for some tube Neanderthals! 
One should be able to do the same to the Cit II by removing the short local plate/grid degeneration loops on the 12BY7 driver tubes and skipping the global loop. The 12BY7 driver does have about twice the gm as the 6AU6 drivers in the SP-10, so some weak local degen. might still be called for to roughly match up the driver stage gain. Also, the KT88 ot 6HD5 outputs have 2x or 3x the gm of a 6V6, but that is mostly needed for the higher current drive into a lower primary Z OT. ( so close to a wash there )
One should be able to do the same to the Cit II by removing the short local plate/grid degeneration loops on the 12BY7 driver tubes and skipping the global loop. The 12BY7 driver does have about twice the gm as the 6AU6 drivers in the SP-10, so some weak local degen. might still be called for to roughly match up the driver stage gain. Also, the KT88 ot 6HD5 outputs have 2x or 3x the gm of a 6V6, but that is mostly needed for the higher current drive into a lower primary Z OT. ( so close to a wash there )
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I have a couple:
The Vixen: Two channel design that uses the 807 final to produce ~32W @ 1.0KHz. The original spec was for 26.5W, but with the grid drivers it hits 32W since it can slip a bit into Class AB2. Uses 30W, Hammond OPTs. If you don't want plate top caps, use 6L6 finals instead (The 807 is identical to the 6L6, just repackaged as an RF type instead of audio.) Other 6L6-oids also work without modification, like the 6BG6 -- either a 6L6 with a top cap, or an 807 with an octal base. (Originally intended as a TV horizontal deflection PA.)
Le Renard: Nearly 40W from 6BQ6GA or 6BQ6GTB finals. Monoblock design. The loadline comes in at 37.6W, but I'm not getting quite that much since I used a junk box OPT -- an old Stancor with 4K4 (P-2-P) primary and 4, 8, 16, 125, 250R secondaries. Looks like originally designed for Class A 6L6s, as the primary load looks right, and included two tertiary windings that look to be providing UL operation since the 6L6 has a low voltage screen (300V specced). Rated for 30W, and that's what I get at 1.0KHz. Busting the PD spec doesn't have any delterious effects when using the 6BQ6s as audio finals, which isn't the same sort of "brick on the key" operation that horizontal deflection duty is, so the ratings from the spec sheet are very conservative.
If you don't want top caps, the design works as is with 6AV5 finals that don't have top caps. Another TV HD type.
Some notes on using Antec OTs and TV Sweep tubes. The Antec OTs are toroidal, so need well matched DC from the tubes. The Cit II circuit does provide for DC balancing conveniently. And of course KT88 tubes can be purchased as matched. Using an EI type OT would still be safer for long term DC drift. ( or a DC servo for the toroidal OT )
With TV Sweeps you won't find matched tubes around. But the 6HD5 tubes were mostly all produced in the same Raytheon Pennsylvania factory. ( some Japanese ones too apparently, keep to one type or the other ) They are not wildly scattered in characteristics as many other multi sourced TV Sweeps are. Still, an AC gain matching pot for the driver stage would be a good idea. ( the Cit II circuit could just put a limited range pot in the short local Fdbk for one of the 12BY7 drivers )
----------------------
HK Citation V, their version of the Mullard 5-20:
Notice the UL connections are gone. I think this came from an RCA manual originally. How to use the new 7581 tube.
With TV Sweeps you won't find matched tubes around. But the 6HD5 tubes were mostly all produced in the same Raytheon Pennsylvania factory. ( some Japanese ones too apparently, keep to one type or the other ) They are not wildly scattered in characteristics as many other multi sourced TV Sweeps are. Still, an AC gain matching pot for the driver stage would be a good idea. ( the Cit II circuit could just put a limited range pot in the short local Fdbk for one of the 12BY7 drivers )
----------------------
HK Citation V, their version of the Mullard 5-20:
Notice the UL connections are gone. I think this came from an RCA manual originally. How to use the new 7581 tube.
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Citation V . . . an interesting amplifier
I think the 7581 Beam Power is similar to the 6L6GC Beam Power. Only the GC has the plate dissipation of the 7581; 30 Watts.
All the other 6L6 versions plates can Not take 30 Watts.
The 7581A plate can dissipate 35 Watts.
I think the Mullard 5-20 used EL34 pentodes; 25 Watt maximum plate dissipation.
If someone wanted to try replacing the EL34 pentode with a beam power tube, the KT77 fits the bill perfectly.
The 7408 plate dissipation is 14 Watts. That 7408 amplifier in Post # 36 is a Harmon Kardon A300 assembled, or K300 (Kit, I built one)
So many things to try.
I think the 7581 Beam Power is similar to the 6L6GC Beam Power. Only the GC has the plate dissipation of the 7581; 30 Watts.
All the other 6L6 versions plates can Not take 30 Watts.
The 7581A plate can dissipate 35 Watts.
I think the Mullard 5-20 used EL34 pentodes; 25 Watt maximum plate dissipation.
If someone wanted to try replacing the EL34 pentode with a beam power tube, the KT77 fits the bill perfectly.
The 7408 plate dissipation is 14 Watts. That 7408 amplifier in Post # 36 is a Harmon Kardon A300 assembled, or K300 (Kit, I built one)
So many things to try.
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Woops, I see the Cit II already has an AC balance pot in the driver loads.
Well, If I were building any of these Amps I would venture to try the cheap 6HD5 output (already have it's cousin 6HJ5 ) with a lower Zpri OT and lower B+. One could even set them up for UnSet type drive, which can give you Xmitter type quality triode curves, without the usual HV or high Z OT expenses.
6HJ5/6HD5 with UnSet drive:
The 6197 driver candidate:
and 6197 in triode:
The Mil GE 6197 tubes I got in a sealed box are so well matched you can skip curve tracer matching.
While 12BY7 tubes I've tested were very un-matched. I've found that the 12BY7 support structure does not have enough anchor points to the Mica to keep the plates parallel. A TOO cheap TV video tube. Oddly, the 6197 tubes are cheap now, since they never found their way into Audio equipment.
Well, If I were building any of these Amps I would venture to try the cheap 6HD5 output (already have it's cousin 6HJ5 ) with a lower Zpri OT and lower B+. One could even set them up for UnSet type drive, which can give you Xmitter type quality triode curves, without the usual HV or high Z OT expenses.
6HJ5/6HD5 with UnSet drive:
The 6197 driver candidate:
and 6197 in triode:
The Mil GE 6197 tubes I got in a sealed box are so well matched you can skip curve tracer matching.
While 12BY7 tubes I've tested were very un-matched. I've found that the 12BY7 support structure does not have enough anchor points to the Mica to keep the plates parallel. A TOO cheap TV video tube. Oddly, the 6197 tubes are cheap now, since they never found their way into Audio equipment.
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