I found the Baby Ongaku schematic - many thanks for that. The usual maximum a-k voltage for the classic 2a3 is listed as 250v, and it looks like the a-k voltage is 250v here, with -50v bias. And thanks for the 300b data too.
andyjevans,
The Baby Ongaku schematic does have 350V B+ marked in two places:
720VCT (360V/side) x 1.414 = 509V peak per side.
509V peak drives a 5V4-GT with a 25V drop, which drives a smallish 0.68uF cap (a modified choke input, that has a very small cap on the rectifier output).
The purpose of the 0.68 was to adjust the B+ to be 350V, and can be changed to get 350V.
The schematic marks the B+ as 350V.
But there is a small drop from that to the 2A3 plate, due to the primary of the DS-025 (primary DCR x 50mA).
Suppose the DS-025 primary DCR is 250 Ohms (10% of the primary impedance).
That is 250 Ohms x 0.050A = 12.5V.
Plenty of B+ in that amplifier.
It is interesting to note that for a push pull 2A3 amplifier, the plate to filament voltage is 300V, the bias is -62V, and the current is 40mA per 2A3 (80mA total). But that is for Class AB1 operation.
Totally different than a single 2A3 single ended, which has to be Class A1.
The Baby Ongaku schematic does have 350V B+ marked in two places:
720VCT (360V/side) x 1.414 = 509V peak per side.
509V peak drives a 5V4-GT with a 25V drop, which drives a smallish 0.68uF cap (a modified choke input, that has a very small cap on the rectifier output).
The purpose of the 0.68 was to adjust the B+ to be 350V, and can be changed to get 350V.
The schematic marks the B+ as 350V.
But there is a small drop from that to the 2A3 plate, due to the primary of the DS-025 (primary DCR x 50mA).
Suppose the DS-025 primary DCR is 250 Ohms (10% of the primary impedance).
That is 250 Ohms x 0.050A = 12.5V.
Plenty of B+ in that amplifier.
It is interesting to note that for a push pull 2A3 amplifier, the plate to filament voltage is 300V, the bias is -62V, and the current is 40mA per 2A3 (80mA total). But that is for Class AB1 operation.
Totally different than a single 2A3 single ended, which has to be Class A1.
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I had another look at the Baby Ongaku schematic and did some calculations with Ohms law to check the currents and voltages. Seems I misread some of the figures first time since they were very small and hard to read. The 2a3 is biased at 50v on the cathode and 50mA. There is indeed 350v going into the OPT and if your ballpark calculation of the primary DCR is correct then there should be around 337v on the plate. So plate to cathode voltage looks like around 287v. This is higher than the maximum 250v in the WE data, but the dissipation is around 14.5W, which is within the 15W maximum in the WE data. I have to presume that this 250v max can be exceeded a little if the dissipation is within 15W. The data does include 300v p-k for PP operation as you say.
Does this all sound correct? The point I'm trying to establish is whether there's some leeway to increase the p-k voltage a little above 250v if the dissipation is within limits? This would help in decision making with an OPT gapped for 50mA.
I'm aware of the JJ 2a3-40 but I'm not going there since I already have some standard 2a3 and 300b tubes. But thanks for bringing that up, good point.
Does this all sound correct? The point I'm trying to establish is whether there's some leeway to increase the p-k voltage a little above 250v if the dissipation is within limits? This would help in decision making with an OPT gapped for 50mA.
I'm aware of the JJ 2a3-40 but I'm not going there since I already have some standard 2a3 and 300b tubes. But thanks for bringing that up, good point.
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Yes, the numbers on the schematic were very hard to read.
As I said, the 2A3 is specified for push pull service, with 300V.
If it can stand 300V in push pull, it can stand 300V in single ended.
The voltage is not the issue, but just as you have noted, the plate dissipation is the issue.
300V x 0.050A = 15 Watts
WE specifications, and tube data sheets, give some suggested operating conditions, but not the only good conditions of operation.
I would be very surprised if Gordon Rankin ever broke the rules on tube specifications.
As to output transformer gapping, and maximum DC current, no two manufacturers make the same tradeoff of things like:
Primary inductance
Lowest frequency before saturation
Highest power at a given low frequency before saturation
Primary DCR
Etc.
Any good quality output transformer should keep you happy.
Otherwise, you have to purchase the very best that is made (only a few manufacturers do that, and they want $$$).
As I said, the 2A3 is specified for push pull service, with 300V.
If it can stand 300V in push pull, it can stand 300V in single ended.
The voltage is not the issue, but just as you have noted, the plate dissipation is the issue.
300V x 0.050A = 15 Watts
WE specifications, and tube data sheets, give some suggested operating conditions, but not the only good conditions of operation.
I would be very surprised if Gordon Rankin ever broke the rules on tube specifications.
As to output transformer gapping, and maximum DC current, no two manufacturers make the same tradeoff of things like:
Primary inductance
Lowest frequency before saturation
Highest power at a given low frequency before saturation
Primary DCR
Etc.
Any good quality output transformer should keep you happy.
Otherwise, you have to purchase the very best that is made (only a few manufacturers do that, and they want $$$).
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i read somewhere that the sweet spot for a 2A3 is 250volts plate at 60mA for plate dissipation of 15 watts,-45v grid bias, i made a single ended amp with that operating point for the 2A3 using Shuguang triodes, amp sounded fabulous....
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Tony,
That is the classic 2A3 SE operating condition. It also lists using a 2.5k primary.
The 300B classic setup is 300V, 60mA, -61V bias, 3.5k primary.
That gives 6 Watts at 5% THD.
But WE gives many other valid setups, in their Original 300B datasheet.
That is the classic 2A3 SE operating condition. It also lists using a 2.5k primary.
The 300B classic setup is 300V, 60mA, -61V bias, 3.5k primary.
That gives 6 Watts at 5% THD.
But WE gives many other valid setups, in their Original 300B datasheet.
yes, lots to choose from as far as 300b is concerned....https://frank.pocnet.net/sheets/084/3/300B.pdf
No sweet spot actually. A certain sweet spot may not be so sweet for Mr. X (plus his system and listening habit + his environment).
Plate voltage for my 300b is 367 volt cathode at 63 volt 50ma...that's very sweet for me.
Regards
Plate voltage for my 300b is 367 volt cathode at 63 volt 50ma...that's very sweet for me.
Regards
A Master was asking his student to clap his two hands together and describe the sound. Then he asked: "What is the sound of one hand?".
euro21,
I like the setup in post #139 better. I still don`t like the input transformer and the plate choke.
Tubelab_com,
the driver arrangement for the single ended 300B constituting such a heavy load is a challenge. My choice would be a grounded cathode amplifier with high amplification factor and a lowish plate load impedance (but the plate resistance could be lower than that of the classic WE 310A).
A high plate load impedance does not give low distortion as widely believed.
I like the setup in post #139 better. I still don`t like the input transformer and the plate choke.
Tubelab_com,
the driver arrangement for the single ended 300B constituting such a heavy load is a challenge. My choice would be a grounded cathode amplifier with high amplification factor and a lowish plate load impedance (but the plate resistance could be lower than that of the classic WE 310A).
A high plate load impedance does not give low distortion as widely believed.
Well, in theory you use a high mu input tube with low Rp. The only problem is getting one to sound really good which isn't as obvious as the theory might led you to believe. There are contenders like D3A and C3g which many like, but which I personally haven't heard. Plus a lot of pentodes in triode or pentode mode. Take your pick.
What I did was throw a bunch of input tubes at the 300b and listen to them. When I did that I liked the EL33 in triode and the 10Y. This then meant I needed a 1:2 step-up for the EL33 and more like a 1:3 or 1:4 for the 10Y. So I then set about looking for good sounding SUTs. Only one of several ways to configure a 300b amp, but I was directed the whole way by the actual sounds I was hearing.
What's your reason for saying a high plate load impedance does not give low distortion? My experience has been that more henries give a cleaner sound, allowing for other factors to be optimised.
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i am winding plate chokes for the 5842 direct drive to a 300b, the power transformer need reworking too.....
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5842
I assume that is what you are talking about, there have been many types discussed in this thread🙂 BTW there is no h in wimpy😉
I assume that is what you are talking about, there have been many types discussed in this thread🙂 BTW there is no h in wimpy😉
Quoting Sakuma san himself :
"The most important goal is not 'Power', but 'Energy' and 'Frame of tone'."
and
"To get the best sound from a tube, even if the proper match is 5K, we will try 7K. And we try these "mismatched" values with input transformers and other parts."
Schematic of his 300B SE from 845/300B SE power amplifier for Lowther driver units.
Attachments
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Yes, thats true for those who have heard and UNDERSTOOD the difference.
A 5842 driver tube will never give that authority, regardless of what current you run it.
Its just too tiny tube to drive something with authority.
The result isn't energetic sound but something that sounds far away some roadblocks.
A 5842 driver tube will never give that authority, regardless of what current you run it.
Its just too tiny tube to drive something with authority.
The result isn't energetic sound but something that sounds far away some roadblocks.