Bombacaototal wrote: ↑Mon Sep 20, 2021 3:45 pm
I've been a bit on a transformer nightmare ride which I want to share and also have a few questions for the members.
I was an avid user of Classic Tone transformers, and always prefered the vintage spect 320-0-320 HV taps (versus the 345-0-345 present as an added option in a few of their transformers). With mains voltage of approx. 240V and using the PT 240V tap, I usually get around 455V in my Twin and Bassman 6L6 amps.
With the advent of CT going out of production I decided to custom order a set from a manufacturer in the Netherlands (won't name names unless someone specifically asks). I ordered a paper wound vintage Fender Twin replica with 120, 230, 240 mains and vintage spec 320-0-320.
I received the transformer poorly packadged and attached is the situation I was comfronted with. Basically the transformers were sent unwrapped in a polysterine enclosure, which reacted to the materials used and basically glue onto the transformer. The builder offered me to send them to a business partner in the UK who properly cleaned them, to a reasonably good state and gave me a small refund as a gesture of good will. At least this issue was sorted, and the manufacturer was helpful in sorting it.
I just finished building the amp and I am getting only between 415 and 420V on the 6L6 plates. I contacted the manufacturer who wrote the following
320VAC rectified with solidstate diodes gives 450V. This 450V will when you put it from standby to play position drop to 423V because the four powertubes draw 30mA each. The 423V will drop about 5V further down through the Outputtransformers Ohms resistance to about 418V. So your 417V measurement is very accurate.
It is simple math. 320 x1,414 = 450V without load. 450 x 0,94 = 423V under load in Class AB. 4-5V losses over the primairy winding of the OT. These are laws that you cannot change. They will apply on every tube amplifier.
I went on to ask why those old fender transformers would usually read between 440V and 460V and have 320-0-320? Below is his answer
What can be is that they give a spec for 320V AC under load. That means that their spec unloaded must be around 340V AC
So my question is, how were the actual spec on those old Fender transformers? Is it 320V loaded or unloaded? Is there anything that can be done on my case? Is there any obvious shortcomings of such low plate voltage besides lower than 100W power?
I have done several 100 watt builds with the Hammond Twin Reverb replacement transformers 290FX and for what its worth they read about 425v on the plates once the amp is loaded with tubes. No complaints from customers on the sound. If you bias the tubes a little hotter you can make up the power loss at idle.
Hammond also makes a nice 373fx transformer that I have used on several 100 watt builds and it has become my personal favorite! Super quite and plate voltage is a little higher than the 290fx and the 373fx doesn't run hot like the 290fx does.
Guy77 wrote: ↑Mon Sep 20, 2021 5:52 pm
I have done several 100 watt builds with the Hammond Twin Reverb replacement transformers 290FX and for what its worth they read about 425v on the plates once the amp is loaded with tubes. No complaints from customers on the sound. If you bias the tubes a little hotter you can make up the power loss at idle.
Hammond also makes a nice 373fx transformer that I have used on several 100 watt builds and it has become my personal favorite! Super quite and plate voltage is a little higher than the 290fx and the 373fx doesn't run hot like the 290fx does.
Cheers
Guy
Thanks a lot for the input Guy, and interesting to know about your experience with these replacement transformers and the low reading similar to mine.
Today I had about 243V from the wall and was getting about 420V on plates. I biased mine a touch above 40mA (via the 1 ohm testing probes). I usually like these amps right around 55% (not sure where I will land with the lower voltages yet tho). Where do you set yours?
Guy77 wrote: ↑Mon Sep 20, 2021 5:52 pm
I have done several 100 watt builds with the Hammond Twin Reverb replacement transformers 290FX and for what its worth they read about 425v on the plates once the amp is loaded with tubes. No complaints from customers on the sound. If you bias the tubes a little hotter you can make up the power loss at idle.
Hammond also makes a nice 373fx transformer that I have used on several 100 watt builds and it has become my personal favorite! Super quite and plate voltage is a little higher than the 290fx and the 373fx doesn't run hot like the 290fx does.
Cheers
Guy
Thanks a lot for the input Guy, and interesting to know about your experience with these replacement transformers and the low reading similar to mine.
Today I had about 243V from the wall and was getting about 420V on plates. I biased mine a touch above 40mA (via the 1 ohm testing probes). I usually like these amps right around 55% (not sure where I will land with the lower voltages yet tho). Where do you set yours?
Hi. I actually set my bias the same why you do at about 55% .
Bombacaototal wrote: ↑Mon Sep 20, 2021 3:45 pm
I've been a bit on a transformer nightmare ride which I want to share and also have a few questions for the members.
I was an avid user of Classic Tone transformers, and always prefered the vintage spect 320-0-320 HV taps (versus the 345-0-345 present as an added option in a few of their transformers). With mains voltage of approx. 240V and using the PT 240V tap, I usually get around 455V in my Twin and Bassman 6L6 amps.
With the advent of CT going out of production I decided to custom order a set from a manufacturer in the Netherlands (won't name names unless someone specifically asks). I ordered a paper wound vintage Fender Twin replica with 120, 230, 240 mains and vintage spec 320-0-320.
I received the transformer poorly packadged and attached is the situation I was comfronted with. Basically the transformers were sent unwrapped in a polysterine enclosure, which reacted to the materials used and basically glue onto the transformer. The builder offered me to send them to a business partner in the UK who properly cleaned them, to a reasonably good state and gave me a small refund as a gesture of good will. At least this issue was sorted, and the manufacturer was helpful in sorting it.
I just finished building the amp and I am getting only between 415 and 420V on the 6L6 plates. I contacted the manufacturer who wrote the following
320VAC rectified with solidstate diodes gives 450V. This 450V will when you put it from standby to play position drop to 423V because the four powertubes draw 30mA each. The 423V will drop about 5V further down through the Outputtransformers Ohms resistance to about 418V. So your 417V measurement is very accurate.
It is simple math. 320 x1,414 = 450V without load. 450 x 0,94 = 423V under load in Class AB. 4-5V losses over the primairy winding of the OT. These are laws that you cannot change. They will apply on every tube amplifier.
I went on to ask why those old fender transformers would usually read between 440V and 460V and have 320-0-320? Below is his answer
What can be is that they give a spec for 320V AC under load. That means that their spec unloaded must be around 340V AC
So my question is, how were the actual spec on those old Fender transformers? Is it 320V loaded or unloaded? Is there anything that can be done on my case? Is there any obvious shortcomings of such low plate voltage besides lower than 100W power?
I have done several 100 watt builds with the Hammond Twin Reverb replacement transformers 290FX and for what its worth they read about 425v on the plates once the amp is loaded with tubes. No complaints from customers on the sound. If you bias the tubes a little hotter you can make up the power loss at idle.
Hammond also makes a nice 373fx transformer that I have used on several 100 watt builds and it has become my personal favorite! Super quite and plate voltage is a little higher than the 290fx and the 373fx doesn't run hot like the 290fx does.
Cheers
Guy
I don't understand the answer you got. What would be the point in giving the output voltage of a transformer without load?
If you buy a 12V 1A transformer you get 12V at 1A current flow and know it is a 12VA transformer. When no current is flowing the output voltage is higher.
Normaly transformer voltages are given at a certain current rating. Usualy the voltage and current so the load matches the transformer impedance giving max output power.
6,3V @ 5,5A
320V @ 450mA
Etc.
You realy don't know anything if you only know the output voltage without load.
What if you specified the filament supply at 6,3 volt and they wind your transformer for that voltage without taking in account any load. You would end up with 4,5 Volt or so under load. Its just stupid!
Thanks for the input Richard, and I agree 100%, it does not make sense the unloaded spec. And I have gone back through the exchanges with the builder and at no point the loaded/unloaded conversation was brought up. Additionally I asked for as close as possible replica of those "vintage" Fender Black Face Twin transformer sets, but it seems builder was not familiar with them, which is a bit surprising. It is a bit of a bummer to be honest, might resort on more well established manufactureres moving forward.
I think at 2.1K OT impedance and the 417V plates I am probably at around 80W output? ?I have adjusted the dropping resistors to have the pre amp and power amp voltages higher, closer to what I'd expect. Besides the lower output anything else I should expect due to the lower plates voltage?
Richard1001 wrote: ↑Tue Sep 21, 2021 8:00 pm
I would not order another transformer from this company. Was it FNS, AEE or ...?
Yes I agree. Since your are asking and just for future reference and transparency the builder did point out that I might get lower output then 100W, but as Fender Twin Reverb is rated 85W and not 100W to be honest I didn't think much of it
This was his remark
The OT can be done with 2,1K. I want to point outh that with a higher primairy impedance and only 325V AC on the PT you will frift away from 100 Watts quite a bit. But we can do it no problem.
And this was my answer
noted with thanks in regards to the 320V-0-320V and OT 2.1K and the output power. The original amp I am replicating used a late 60s Blackface Twin Reverb set and I am really trying to keep as true to original as possible.
Take as you will, in my mind my goal was clear enough and I would expect someone selling transformers to be well versed with the "classics". In retrospect I should have investigated more his remark about the power rating and maybe posed some questions which might have prevented this. It is easy in retrospect anyway, but it's a learning lesson for the next time
We had a similar chat in regards to choke whereby I pointed out my desire to keep it true to the original Twin and given his was really different spec I luckily decided to source this elsewhere
Anyway, for reference the maker is named Marble Amps
One more thing I have been thinking of, those vintage fender PT were speced to 117V. The reference voltage in Japan is in the range of 100V. Could an approximate 17V difference in the mains voltage yield in such a difference in the B+. As per the Jap schematic B+ on 002 was reading 378V. If the Twin is meant to rest 460V there is almost 100V difference there
Richard1001 wrote: ↑Tue Sep 21, 2021 8:00 pm
I would not order another transformer from this company. Was it FNS, AEE or ...?
Anyway, for reference the maker is named Marble Amps
Yes, that's FNS. Very peculiar since Marble amps are mostly modified Fender tweed clones. And it woudn't be that hard to find the blackface transformer specs online.
Bombacaototal wrote: ↑Tue Sep 21, 2021 9:07 pm
One more thing I have been thinking of, those vintage fender PT were speced to 117V. The reference voltage in Japan is in the range of 100V. Could an approximate 17V difference in the mains voltage yield in such a difference in the B+. As per the Jap schematic B+ on 002 was reading 378V. If the Twin is meant to rest 460V there is almost 100V difference there
It is very easy to calculate. The Japanse schematic shows the input voltage and output voltage of the powertransformer. You can therefor calculate the turn ratio and use this to calculate the output voltage when het mains voltage is 117V.
The power your amp will put out is not clear based on the highvoltage in idle alone. It is important that the transformer can deliver the power needed (in other words: the voltage doesn't drop much further under full load) Since you have no specs on the transformer, you can only measure the performance.
Richard1001 wrote: ↑Tue Sep 21, 2021 8:00 pm
I would not order another transformer from this company. Was it FNS, AEE or ...?
Anyway, for reference the maker is named Marble Amps
Yes, that's FNS. Very peculiar since Marble amps are mostly modified Fender tweed clones. And it woudn't be that hard to find the blackface transformer specs online.
I agree, really disappointing, and the worst of all is that I bought 2, which means I have another PT in same situation. I am seriously thinking of scrapping it and getting a new one
Bombacaototal wrote: ↑Tue Sep 21, 2021 9:07 pm
One more thing I have been thinking of, those vintage fender PT were speced to 117V. The reference voltage in Japan is in the range of 100V. Could an approximate 17V difference in the mains voltage yield in such a difference in the B+. As per the Jap schematic B+ on 002 was reading 378V. If the Twin is meant to rest 460V there is almost 100V difference there
It is very easy to calculate. The Japanse schematic shows the input voltage and output voltage of the powertransformer. You can therefor calculate the turn ratio and use this to calculate the output voltage when het mains voltage is 117V.
The power your amp will put out is not clear based on the highvoltage in idle alone. It is important that the transformer can deliver the power needed (in other words: the voltage doesn't drop much further under full load) Since you have no specs on the transformer, you can only measure the performance.
Ah true, I missed the input voltage. So 102V mains will give 378V B+, at 117, it should be around 433.6V
I can probably get spot that if I use the 230V tap on the PT given the mains at 240V, but really limits my amp to UK operation
I will do some research on how to measure the actual output performance of my amp
