SE31 and PP amps with 13E1.

Here are two more schematics for 13E1 tubes used for Single Ended amps and Push Pull amps.
These schematics were drawn up to suit the use of OP2 which I sold to a lucky customer who
had twenty 13E1 tubes which he wanted to be able to use.
Now OP2 could have been used with air gapped C-cores to suit 1 x 13E1 in SE mode or with
un-gapped C-cores to suit 2 x 13E1 in PP mode. I gave both options to my customer and he
chose the PP option. I think he chose wisely, because my experience with 8585 and other PP
amps I have made tells me that not much is wrong with PP operation where there is a large
percentage of total power in class A1.

Schematic 1. SE31, similar to SE32, but different OPT, lower B+.

Schematic 1 uses an OPT numbered OP2 which I did have for sale, but now have sold.
After carefully analyzing the numbers of turns and winding configuration for OP2,
I concluded it could be used either for SE or PP. I thought its best application meant the use
of the shown Ea, Ek, and Eg2 and OPT turn ratio. DIYers need to realize that if they ever
find a given OPT for which they want to use as a basis for an amp design, there is usually
only a very limited range of options if they are to match the OPT to something other than for
what it may have been intended which was not designed specifically for a given tube and its
operating conditions. I've also seen very poor design practice used by mass makers.
Notice the the fixed Eg2 slightly higher because the amount of CFB is 17.25%, thus there is
no risk of Ia cut off with the lower Eg2, which is raised to +200Vdc. The intended OPT for
this amp has much lower RLa of 1k2 than 2008 amp with 2k8, so Ia is much higher, Ea lower.

Schematic 2. PSU for SE31 with lower B+.

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For classic Class AB1 Push Pull operation with 13E1, giving plenty of class A :-
Schematic 3. PP 13E1

The buyer of OP2 settled on PP use of 13E1 after I designed this schematic.
The schematic basics are similar to other PP circuits such as 8585 and 300W amps elsewhere at
this website. Instead of using 2 x 13E1, anyone could a pair of KT120, or quad of other octal tubes
although Eg2 would need to be higher, and biasing methods revised. In 20 years when all stocks of
13E1 are gone, a change to 4 x KT88, 6550, EL34, 6L6GC, etc could be used with the same basic
power supply and OPT. The input and driver amp has what I now think is the best PP input driver
combination for where the maximum voltage needed to drive OP tubes is substantial, say over 50Vrms
per grid.

Schematic 4. PSU for PP 60W amp with 13E1 :-


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