SE31 and PP amps with 13E1.
Here are two more schematics for use with 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 20 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+.


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

Fig 7.
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.

Fig 8. The power supply for PP 60W amp with 13E1 :-

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To SEUL 25, 1997

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