![[-]](https://theultimatetone.com/images/collapse.png "[-]")
09-10-2026, 01:15 PM
Hi Guys
Transformers are expensive, so I always try to design things not to stress them very much. The idea of using an output transformer to limit the bandwidth at full power seems like a sure way to burn them out - and it is! When I was servicing amps I replaced quite a few in Peavey and Marshall amps, both of which have under-sized OTs, especially Peavey who winds their own.
Another factor for me is that I do not play as loudly as many other guys even given an opportunity to play loud in a place where it might be expected. So, I liked the sound I had playing well-below the power rating of the OT where bandwidth is fairly unrestricted. Later, when I developed Power Scaling and was selling amps I wanted the tone to be consistent from quiet to loud and I used OTs rated at or above the power rating of the amp.
EI laminated OTs have innate distortion specific to their broad-stroke design. This can be minimised by going to exotic output stages using some amount of cathode drive, where both the plate and cathode drive windings on the OT. The extreme example of this is Frank McIntosh's unity-coupled output transformer where the winding ratios are 1:1 for the plate and cathode windings. This requires a quite different drive scheme since the entire output tube "floats" with the signal.
Wanting to keeps things more conventional, the over-rated OT approach is the least expensive and simplest circuit-wise with no changes in the latter. For a cleaner tone, the way to go is with toroidal OTs. Again, the schematic does not change. The choice of OT was limited at the time I wanted to try this out, but a friend I was working with found a few newer suppliers in the area and these companies had better pricing. The results were good and the second new supplier (SumR, now retired) turned out to be fantastic at designing OTs.
I was able to switch to toroidal PTs first, since I still had a lot of my custom OTs (those lead Hammond to release their "easy tap" OTs). For a while my amps had this hybrid transformer group until I switched to toroidal OTs as well.
As I have repeated many times, toroidal OTs do not distort musically.
Other builders, such as Aguillar, used toroidal OTs from Plitron, his being for a 400W bass amp. The OT is large, heavy and expensive and his amps sounded great. How many players clipped their Aguillar bass amps? Probably a lot.
My transformer guru made me some very compact OTs using special core materials. My PTs had gotten rather huge since they were the same as for the kits and the heater windings became more and more massive. This increased the size difference between the PT and OT, so I asked for much over0rated OTs. My nominal 50W OT is actually designed as a 100W and it could be used at that power level forever.
The only problem I had with any of the toroidal transformers were with bias windings. This is a "light" winding as far as its required loading goes, so it has very thin wire compared to the plate winding and especially compared to the heavy heater winding. A couple of PTs from the first toroidal supplier opened, rendering the PT inadequate or necessitating the addition of a separate bias PT. It was because of those failures that I asked SumR about over-sized windings.
Another factor that lead to my PTs being larger was specifying a lower temperature rise. PTs of all kinds generally have a temp rise of 40C at full output. Some smaller PTs run as hot as 60C. The higher the temp rise the higher will be radiated EMI. Reliability may or may not be effected, but in any case there is assured longevity with lower-temperature operation.
Regarding the OT tone, doing the swap within the same amplifier is the only real way to assess the difference. Nominally, a three-pole switch could switch the plate winding ends and the speaker output. Both OTs should have the same Raa value. I did this testing a prototype OT designed by Menno Vander Veen back in 2004 or so. His OT was pretty small and he used a saturable core material. The test amp was a Fender with its usual under-size OT.I played through them at a normal practice level that would not have pushed them to clipping or limiting. The toroid was a bit cleaner than the EI, as expected.
Since my OTs were designed more conventionally AND specified for full power bandwidth at the rating of the amp it was to be used in, it always met all my goals for consistent tone from quiet to loud. A consistent goal has been to never stress the OT so it never has to be replaced. I use different connections than other designers and apply my alchemy to other parts of the circuit. The clean OT - whether EI or toroid - becomes a Pillar of Tone.
Transformers are expensive, so I always try to design things not to stress them very much. The idea of using an output transformer to limit the bandwidth at full power seems like a sure way to burn them out - and it is! When I was servicing amps I replaced quite a few in Peavey and Marshall amps, both of which have under-sized OTs, especially Peavey who winds their own.
Another factor for me is that I do not play as loudly as many other guys even given an opportunity to play loud in a place where it might be expected. So, I liked the sound I had playing well-below the power rating of the OT where bandwidth is fairly unrestricted. Later, when I developed Power Scaling and was selling amps I wanted the tone to be consistent from quiet to loud and I used OTs rated at or above the power rating of the amp.
EI laminated OTs have innate distortion specific to their broad-stroke design. This can be minimised by going to exotic output stages using some amount of cathode drive, where both the plate and cathode drive windings on the OT. The extreme example of this is Frank McIntosh's unity-coupled output transformer where the winding ratios are 1:1 for the plate and cathode windings. This requires a quite different drive scheme since the entire output tube "floats" with the signal.
Wanting to keeps things more conventional, the over-rated OT approach is the least expensive and simplest circuit-wise with no changes in the latter. For a cleaner tone, the way to go is with toroidal OTs. Again, the schematic does not change. The choice of OT was limited at the time I wanted to try this out, but a friend I was working with found a few newer suppliers in the area and these companies had better pricing. The results were good and the second new supplier (SumR, now retired) turned out to be fantastic at designing OTs.
I was able to switch to toroidal PTs first, since I still had a lot of my custom OTs (those lead Hammond to release their "easy tap" OTs). For a while my amps had this hybrid transformer group until I switched to toroidal OTs as well.
As I have repeated many times, toroidal OTs do not distort musically.
Other builders, such as Aguillar, used toroidal OTs from Plitron, his being for a 400W bass amp. The OT is large, heavy and expensive and his amps sounded great. How many players clipped their Aguillar bass amps? Probably a lot.
My transformer guru made me some very compact OTs using special core materials. My PTs had gotten rather huge since they were the same as for the kits and the heater windings became more and more massive. This increased the size difference between the PT and OT, so I asked for much over0rated OTs. My nominal 50W OT is actually designed as a 100W and it could be used at that power level forever.
The only problem I had with any of the toroidal transformers were with bias windings. This is a "light" winding as far as its required loading goes, so it has very thin wire compared to the plate winding and especially compared to the heavy heater winding. A couple of PTs from the first toroidal supplier opened, rendering the PT inadequate or necessitating the addition of a separate bias PT. It was because of those failures that I asked SumR about over-sized windings.
Another factor that lead to my PTs being larger was specifying a lower temperature rise. PTs of all kinds generally have a temp rise of 40C at full output. Some smaller PTs run as hot as 60C. The higher the temp rise the higher will be radiated EMI. Reliability may or may not be effected, but in any case there is assured longevity with lower-temperature operation.
Regarding the OT tone, doing the swap within the same amplifier is the only real way to assess the difference. Nominally, a three-pole switch could switch the plate winding ends and the speaker output. Both OTs should have the same Raa value. I did this testing a prototype OT designed by Menno Vander Veen back in 2004 or so. His OT was pretty small and he used a saturable core material. The test amp was a Fender with its usual under-size OT.I played through them at a normal practice level that would not have pushed them to clipping or limiting. The toroid was a bit cleaner than the EI, as expected.
Since my OTs were designed more conventionally AND specified for full power bandwidth at the rating of the amp it was to be used in, it always met all my goals for consistent tone from quiet to loud. A consistent goal has been to never stress the OT so it never has to be replaced. I use different connections than other designers and apply my alchemy to other parts of the circuit. The clean OT - whether EI or toroid - becomes a Pillar of Tone.
