Bartolucci output transformers.....
A few years back I bought a pair of Bartolucci outputs from ebay. The paper work with them says they are a model 2....push pull class A-A1. Not sure what the A-1 means? Primary impedance 2500 ohms, Secondary 8 ohms. 30 watts OP. Tubes listed are 6AS7....2A3....6B4....6080....6L6.
Does anybody know of a class A push-pull kit that would work with these outputs?
A few years back I bought a pair of Bartolucci outputs from ebay. The paper work with them says they are a model 2....push pull class A-A1. Not sure what the A-1 means? Primary impedance 2500 ohms, Secondary 8 ohms. 30 watts OP. Tubes listed are 6AS7....2A3....6B4....6080....6L6.
Does anybody know of a class A push-pull kit that would work with these outputs?
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Class AB could work with low power tubes (under 30W in PP of the right type) With a primary of 2.5K possibly Tubelab George Anderson (has a Form on this site) PP PCB for EL84/6BQ5 or 6CW5/EL86 with some component changes, the OPT (output transformer) being one of them.
Tubes and Applications | Tubelab
He specs 3K to 3.6K , but your so close!
He still may have parts kits to populate the PCB, so ask him on this form.
Another option is to parallel output tubes, but your 30W rating rules that out for most affordable tubes. EL34/KT-77 in triode mod will just squeak by as it's affordable and there are many after market PCB's for the Dynaco ST-70 that could support that.
300B tubes run in class A PP might be another option, especially if parallel, but big $ for all those tubes.
Tubes and Applications | Tubelab
He specs 3K to 3.6K , but your so close!
He still may have parts kits to populate the PCB, so ask him on this form.
Another option is to parallel output tubes, but your 30W rating rules that out for most affordable tubes. EL34/KT-77 in triode mod will just squeak by as it's affordable and there are many after market PCB's for the Dynaco ST-70 that could support that.
300B tubes run in class A PP might be another option, especially if parallel, but big $ for all those tubes.
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ehh ... is he not referring to load for each EL84 (which might even be on the low side ?)
2.5K ... is that not more like typical SE load ?
Actually parallel 2A3 would be doable as they are easier to drive than 300B, if you could afford the tubes.
A common Dynaco ST-70 aftermarket circuit (easy if you just buy a PCB board with parts) could be used to drive them.
From my reading single plate 2A3 like the JJ are sonically great. Quite affordable for single plate and well tested from Eurotubes. (From audioassylum forms etc.)
https://www.eurotubes.com/cart/index.php?page=view_products&category_id=3&sub_category_id=24
A common Dynaco ST-70 aftermarket circuit (easy if you just buy a PCB board with parts) could be used to drive them.
From my reading single plate 2A3 like the JJ are sonically great. Quite affordable for single plate and well tested from Eurotubes. (From audioassylum forms etc.)
https://www.eurotubes.com/cart/index.php?page=view_products&category_id=3&sub_category_id=24
ehh ... is he not referring to load for each EL84
2.5K ... is that not more like typical SE load ?
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No the 6CW5/EL86 in PP! for maybe up to 20W/CH. That would be the most affordable.
Another option is Pete Millets Big Red PCB if you could find the right TV tubes to work with 2.5K primary's with only 30W max.
http://www.pmillett.com/dcpp.htm
Maybe George knows!
Another affordable and doable option is 6 (yes six) EL84/6BQ5 as they are affordable and very easy to drive. Run them in Triode mod for sonics and to keep the power level close to 30W/CH. You would get the perfect primary (7.5K) for those tubes run in fixed bias where you could adjust for the best sonics and life.
Again as easiest to drive of (especially affordable and sonically well liked) production tubes , just use the proven Dynaco after market PCB 's out there.
2.5K ... is that not more like typical SE load ?
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No the 6CW5/EL86 in PP! for maybe up to 20W/CH. That would be the most affordable.
Another option is Pete Millets Big Red PCB if you could find the right TV tubes to work with 2.5K primary's with only 30W max.
http://www.pmillett.com/dcpp.htm
Maybe George knows!
Another affordable and doable option is 6 (yes six) EL84/6BQ5 as they are affordable and very easy to drive. Run them in Triode mod for sonics and to keep the power level close to 30W/CH. You would get the perfect primary (7.5K) for those tubes run in fixed bias where you could adjust for the best sonics and life.
Again as easiest to drive of (especially affordable and sonically well liked) production tubes , just use the proven Dynaco after market PCB 's out there.
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If you like EL-34 types (KT-77 is supposed to be nice) they are also doable in PPP as you would end up with the right 5K primary. You would get 15W/CH in Triode mode from a pair so your OPT's would be fine with their rating of 30W running a quad.
They would take more drive so a circuit running KT-88 would be more appropriate like the ones running Dynaco Mark III's. There are some popular affordable PCB boards and kits of parts to populate it from Classic valve design, Triode Electronics, & Tubes4hifi, and maybe others.
Maybe build mono blocks using new production (or used for low cost) Dynaco ST-70 chassis.
They would take more drive so a circuit running KT-88 would be more appropriate like the ones running Dynaco Mark III's. There are some popular affordable PCB boards and kits of parts to populate it from Classic valve design, Triode Electronics, & Tubes4hifi, and maybe others.
Maybe build mono blocks using new production (or used for low cost) Dynaco ST-70 chassis.
is there any special reason why you posted this on Tubelab forum instead of regular tube amp forum
This is not correct. Class A refers to all amplifiers where current flows continuously throughout the entire audio cycle whether they draw grid current or not. All SE audio amps are class A by design.
Class A can be further subdivided into class A1 and class A2.
A class A1 the amplifier is designed such that the output tube's control grid never intentionally draws current. Most SE audio amps are class A1.
A class A2 amplifier is designed such that the control grid can be driven positive and therefore draws current. Usually the output tube will draw grid current for only part of the audio cycle. For an A2 amplifier to be succesful the driver circuitry must be capable of supplying the current drawn by the grid. Transformers, mosfets, or cathode follower drivers are required. Some transmitting tubes were designed such that they must be operated in A2. The 811A, 845, 805 and 833A are examples. Some conventional audio tubes also work quite well in A2. The 45, 300B, and even the 6L6GC are examples.
There is nothing specific about an output transformer that would require class A1 operation. Many people believe that a class A design is "better" than class AB and that a pure triode design is better than a pentode based design. I will agree that there are some technical merits to these beliefs, but they are not universal truths. Most builders state that class A1 is better than class A2. I disagree here. It is true that class A2 is MUCH HARDER to get right, but a well executed A2 design can be excellent.
The OPT's that you have are 2500 ohms plate to plate impedance. This reflects a 625 ohm load impedance to each side. There are very few triodes that can drive this load in class A. The 6AS7 or 6080 is one possibility, but these are not the most linear tube in the tube manual.
Yes, I built a P-P 300B amp about 10 years ago. It used 6600 ohm OPT's and made about 18 WPC if run in class A. 4 X 300B's per channel would work with your OPT, but as stated be a rather expensive solution.
The 6CW5 in the SPP board will work with a 2.5 K OPT and a relatively low B+ voltage around 200 to 250 volts. It will not work in triode class A at that load impedance. It would run in pentode class AB1 and make around 18 to 20 WPC.
If I was asked to make an amp worthy of Bartolucci transformers that produced 30 WPC with a 2500 ohm load, that would be what I would try first since I have one already built up. Sweep tubes are designed to push big currents and that's what's needed with a low impedance load. For 2500 ohms you would want a 18 watt class or bigger tube running on a lowish B+ voltage of 300 to 350 volts. Power output would be dependant on the B+ voltage but there would be no problem getting 30 WPC.
If you are dead set on having a Class A triode amp without spending the $$$+++ on 300B's there are two other possibilities, but some experimenting is needed. You could wire two EL34's or KT88's per side and find a suitable driver board for them. You could also use a pair of large sweep tubes wired in triode mode. Trioded sweep tubes are unstable at high B+ voltages since the screen grids don't like HV, but at 300 volts, most are OK.
I have several different driver boards nearly ready but I can't say if or when they will be ready for sale, since I have been informed my job of 41 years will end soon. I will be moving out of state, disrupting Tubelab activities for an unknown time period.
I have used Pete's P-P driver board and it will drive any tube that you would put in a 30 to 50 WPC amp in class A1 or AB1. It will not do A2 or AB2 without added help.
George, I'm so sorry to hear about your job situation.... after 41 yrs they lay you off!! Maybe you're better of without them? Good luck on future endeavors.
My amp building experience is limited. I have a Golden Tube SE 40 that I have modded by following directions from the GT forum. Also a pair of Dynaco Mk4s modded with VTA boards, from that forum. Limited experience for sure, but I do like building the boards, so I've bought all 3 of your boards for winter projects.
Like I said I bought these Bartolucci transformers a few years back and I must have got a good price on them, or I wouldn't have bought them. 2A3 and 300b are a little rich for my blood. I'd rather get close to that sound for cheap. More fun, too. I'd just like to find a way to use these transformers the best way I can.
Thanks to all for the help, everyone.
My amp building experience is limited. I have a Golden Tube SE 40 that I have modded by following directions from the GT forum. Also a pair of Dynaco Mk4s modded with VTA boards, from that forum. Limited experience for sure, but I do like building the boards, so I've bought all 3 of your boards for winter projects.
Like I said I bought these Bartolucci transformers a few years back and I must have got a good price on them, or I wouldn't have bought them. 2A3 and 300b are a little rich for my blood. I'd rather get close to that sound for cheap. More fun, too. I'd just like to find a way to use these transformers the best way I can.
Thanks to all for the help, everyone.
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slightly off topic, and yet, maybe not
anyway I decided to try and find out how to measure my output trafo
I like this one, if it works
but sounds like something I can manage
in any case, his other stuff is worth a look too
Finding Unknown Values for Output Transformers
please just tell me to move this post, if not ok
anyway I decided to try and find out how to measure my output trafo
I like this one, if it works
but sounds like something I can manage
in any case, his other stuff is worth a look too
Finding Unknown Values for Output Transformers
please just tell me to move this post, if not ok
That is an accepted method, and it will work. You need to find the turns ratio by applying a known voltage to one winding and measuring the other winding(s). The key is accurately measuring the voltages. Use the same or an identical voltmeter to measure the voltage on each winding. Not all AC voltmeters read the same, particularly if there is any distortion on the AC source. My power line voltage runs about 10% distortion. Measure the voltages with the unknown transformer connected. I have found that some transformers can give erroneous readings with no load, so a resistor across the winding being measured helps. Use something large enough to consume less than 1 watt.
I have a rather simple methode of testing unknown OPT's. I DO NOT recommend it for inexperienced people, and it is probably not a good idea in Europe or other 240 volt 50 Hz countries. I put an 8 ohm load on the secondary and plug the primary into line voltage. The meter is reading the secondary voltage across the 8 ohm resistor. The primary impedance is (122/ meter reading)*(122/meter reading)*8. 122 is my nominal line voltage, and 8 is my nominal speaker impedance. The load resistor is 8 ohms because I have several. Anything from 4 to 20 ohms will work.
This works because most tube OPT's see several hundred volts in normal use. 122 volts is far below normal even for a tiny SE OPT from an old radio. There could be some tiny radio or guitar amp OPT's that can't handle 240 volts at 50Hz, so I wouldn't recommend trying this on small OPT's.
I have no problem answering most tube related posts as time permits, but the OP may get more exposure, and maybe more opinions if this was in the general tubes and valves forum, so it is up to him.
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