Low plate , low wattage combo amp ( EL91)

[bepone]

the story is very long and draws its roots from the fact that most small combos sound uninteresting, thin, hollow, boxy, too bassy, muddy etc.. small cabinet built from "random" material creates a lot of negative effects to the sound, more than positive. of course 4x12 or 8x12 cabinet sounds perfect, but can a small portable combo amp that has only a few advantages over others - that is small, light and portable, sound good, really good - always interested me and encouraged a lot of different attempts and iterations to investigate in this direction.

friend of mine added a lot on it with one (too small) MDF cabinet where was 5W s.e. amp installed with some shitty 8" speaker. with trafos from old radio. EL84 tube. the sound was for nothing, useless, short and dead tones.. not because of the amp only, it was the cab and speaker!

i have tried to change mdf front baffle for thin ply, but no change. then i have tried to increase 8" speaker for 12" and put some classy speaker like Alnico blue, or 12-65 celestion.. but sound was always limited, some hollow unpleasant middle was always present.

[bepone]

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this bad mdf one is red one the right. i had in plan to build one low powered "fender" type of amp, few watts, to test some new cabs and ideas (this "simple amp idea" at the end became more and more complicated, so i have finished at low plate dumbleish amp )

about the cabinet.. cabinet was built with 12mm birch plywood cut on cnc, mostly to test the birch ply and thin front baffle (12mm but i wanted to try also thinner, to give more resonance to the front end to keep the sound "alive" . front baffle has middle part with cone shape to the internal center for dispersion of the highs from the speaker centre. in the same time i have built one small cabinet for combo amp and one really oversized 1x12" to keep for the speaker cabinet. small one started like this

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[bepone]

speaker mounted on naked version of the combo cabinet and test was done few days with several tube amplifiers.

sound was completely different from mdf version, it was smaller version of classic full bodied guitar sound from the big 4*12" cabinets! even better was oversized cabinet where sound get another dimension, in the term of prolonged sustain and even more "organic" sound on the clean! prolonged sustain on the clean only with cabinet! people are using effect pedals to get this!

ok this changed my world about importance of the cabinets, and i was sure before that it is important to have the excellent speaker in proper cab, but now im sure to say that at least 40% of total sound from guitar to the speaker falls to the speaker cabinet. what a waste of time is discussion about the pickups and the cables!

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i decided that i don't want to tolex small cab to keep all fast (but did the bigger one in white) and painted acrylic black coating with textured effect. this was a small mistake where i have lost something in overall sound, "liveness" and "sustain" for sure in some percent. but it is what it is. next one when will be, i won't paint , i will just pass some thin shellac for basic protection and coloring.

[bepone]

about the amp

amp was built from scratch from material what i have found in the workshop .. output transformer is project for 2xEL95, 5W, 10kaa, 4, 8, 16 ohms, collecting the dust for the years.. power transformer is toroidal type for random valve preamp (230V/0.12A, 12.6V/1.2A) . can pass. if i power the ECC83s in the preamp on 12V parallel heaters and output tubes in series (6.3V 0.3A/tube max) i'm in the ballpark. output tubes for the start were chosen 12B4A triodes to have better damping factor than small pentodes (to gain some control on the raged speaker impedance) and they have heater option parallel for 12.6V/0.3A.
i needed also bias winding so i wound 18VAC externally on the toroid with some spare wire and isolated with the tape. toroid is edeal for all king of mods

i had some remaining pcbs from old dumbleish project to spend.. first i was thinking to use use only clean channel and with time to add drive if necessary. but finished completely.

material was selected in dumble low plate fashion,, actually Fender classic, 1.5k/22uF cathodes, 100k anodes, with classic Fender tone stack , 270pf, 47nF middle, 100nF bass, 100k slope CF resistor. no R/J switches here, kept all simple.
metal film (ISKRA) on cathodes, modern Vishay PR MF for anodes and for the signal i kept carbon film (Iskra plus other vintage, some pihers) and carbon comp, mostly because i have them only to collect dust. from capacitors selection, for the tone stack i kept Orange drops 6PS series mixed with Arcotronics polypropilenes. At the PI and after i use white ISKRA MKT's and MKP's to find some use in the guitar circuits!

for the presence cap (usually 1uF-2uF tantal) i put Iskra MKT 1uF. For some reason i put also WIMA 470pf FKP in V1b and remained there until now.

for OD i have installed HRM board to have some tone shaping options, other is low plate - 100k on anodes, 1k5 MF res. on cathodes//4u7 uF .

for PI i have mixed Iskra / Piher stuff, for the anodes 110/120K DALE MF. 33nF Iskra PP KNI output coupling caps .

output tubes 12B4A in fixed bias.

front and back plate engraved with cnc, thin V bit

it looked like this

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[bepone]

new and old version. for the grill i found some cane cloth residuals

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[10thTx]

THANKS for sharing this! I love these innovative ideas. Bravo! What a nice looking amp and an interesting idea!

Would you be willing to share the schematic? I've attached an ExpressSCH editable schematic you could edit OR maybe mark up the GIF version if you don't have a PC to edit the SCH version? Also if you could share the voltage on node A on the B+ rail, please?

If you are willing to share a schematic, I'll delete mine on this post to avoid confusion. Thanks!

I would sure love to hear some sound clips or watch a video of this amp!

With respect, 10thtx

[bepone]

thanks 10thTx for kind words!
no problem i will share the schematic i have no commercial interest in this amplifier, it is a hobby use of old & forgotten tubes! i will edit sch file..

at the moment voltage is (too) high for EL91 (230VAC toroid , rectified to cca 300VDC) and it is much better for EL95 (sockets are the same only one wire need to be rewired , g2),
but i have long expertise on EL95 last 5 years, i like it too much, but will be boring to follow again "old route"

[bepone]

but lets go back little bit to the story.. amp had 12B4A triodes in the output and various speakers were installed (Alnico blue, 12-65, Alnico Red Fang Eminence, Celestion Greenback Herritage G12M).. at the end Greenback was installed but sound was too dark..silent ok, for home use but little bit too dark. it was only acceptable for the drive and not clean. triodes are just simply not good for guitar amp output..missing too much presence what pentodes and tetrodes naturally have.

i gave the amp to my friend for 6 months to receive some feedback, but he didn't use clean at all and confirmed my suspicions for the "darkness".. last couple of days i started to check how i can modify the output. other friend just gave to me box of some old radio and TV tubes, EF91, EF95, EL91, EAA91.. wait "EL" prefix is a sign of pentode! what is this? small output pentode 4W from the portable and car radios? possible output in PP 5W? perfect!

i started to drill some holes to accept 7 pin sockets, before were noval 12B4A! OPT 10k is not perfect, PP EL91 searching 15k, but if i connect 16 ohm speaker to the 8 ohm OPT tap i will have 20k, nice again!

so started EL91 journey ..

[bepone]

i have changed arcotronics PP for 6PS in some positions, also in the drive stage for the coupling cap to the drive pot i have put OD 715 polypropilene for experiment, and Iskra MKT's in HRM circuit!
amp now looks like this:

amp1.jpg

amp2.jpg

amp3.jpg

about the sound.. amp is still under monitoring because it is a new thing, new concept, new tubes! like the new wife , all is different and amusing but characteristics are all boosted to better ,clean is really nice, bell-like without harshness, , drive (no PAB only drive) can be from crunch to heavy rock with a lot of gain! actually i dont know what is better sound, clean which is very nice, or drive at max gain /volume, drive and drive volume all maxed, master on half/ where you have really powerfull drive for the heavy chords and solo, but there is no sign of raspyness, fuzz, fizz, all artifacts unwanted are not here! at the volume which is not loud! load for the room and night playing, but not so loud that during the day somebody in neighbourhood will complain!

i was concerned about g2 current so during max loud playing (several h), g2 current was monitored, and looks thats at the limit (260V g2 voltage without signal drops to 235V.. over g2 stopper 10k 2W). this means that 25V over 1k, 2.5mA grid 2 current was increased or in power -> 235*0,0025=0,58W.. and g2 dissipation is 0.6W. so maybe i will increase g2 stoppers to 15k just to protect poor EL91 grids more..

[fred.violleau]

Hey Bepone, you are full of surprises!

Love to hear about different tubes on the power section.
On my Dumble journey that started a few years ago, I bought a ceriatone Dstyle 20w from a friend. I

It was really deceiving, clean sound was ok but very flat and drive was barely what feels like a Dstyle amp for me. It was bragging 20watts and honestly never had an amp with that little output...

It is always an interesting struggle to assemble a low wattage amp that sounds good. I have had good fortune with my amps played in a 2x10 or a 2x12 speaker cab. It seems Bartlet amps are focusing on the same area : getting a cab that vibrates and participate in the sound as much as the amp.

https://youtu.be/BkaCKtUd79M

Do you have a recording to share?

Fred.

[bepone]

ejla fred!
one of my friends bought one cheap pedal, some DIY overdrive which helped to have the D tone from any amp, pluged the LP guitar in in some D type of amp which didn't sound like D type. pedal pressed and arrived really some sweet middle tones, nice ovedrive full of sustain. i didn't beleive! pedal took the authority on the overall sound and make the D- tone! after i have changed the tone stack in the amp, gave touch two more bass , increased tone stack caps from 47nF to 100nF, installed DEEP switch and it was totally different amp!

so what can be in your case, it is a combination of the material used, capacitors and resisitors at the end, and some lead dress!

i was last 10 years biased for using carbon films everywhere, but slowly i'm converting all to metal films, also in the grids what was heresy for me which changing a lot the character of the amp (tighten the bass, adding presence and clarity, for noise i don't care generally but of course noise is very low with MF)

for the amp from the thread, i'm satisfied completely with the amp, nice clean tones, rocky overdrive, no harshness.. although the voltages are little bit high (300VDC supply instead of 250VDC). power measured 4.5 Watts and it is too loud for the home use

but sound from the small cabinet can be better, searching the speaker now to pair with this amp.. Greenback is a little bit too muddy for it.. amp is running to the external speaker cab (closed cabinet with Eminence Red Fang Alnico and really shines there). for the clips i don't have time unfortunately at the moment because i'm going theese days to long journey offshore..will be in march..

[bepone]

p.s. for the Bartels yes i have checked those cabinets, interesting (need to see in reality if is true but it is too expensive to check)

[bepone]

actual schematic with component types and values. there is a lot of MF resistors and polypropylene capacitors so no need to add any "bright" caps over volume pots. it is ok without.

there is nice normal clean, without excess highs and creamy rocky overdrive. in case on the first vol pot 36pF is engaged, clean of course is more present and nice, but OD became more "hairy". it is ok for the one who likes it ( depending of the speaker) , im keeping it off Red Fang has a lot of treble.

no trimmer for the OD input, fixed resistors. slightly changed some values in the amp, drifted from the D classics suited to my taste. 0.22 cap at the PI second grid to the gnd for more bass (it is a 1*12 it needs little bit), tried also anti-blocking distortion 22nF there, but sound is too thin. returned to 0.22.

10k g2 resistors lowered to 2k2, but increased to 6k6 resistor between Ub+ cap and g2 elco cap.

D-EL91 12.01.2022.bmp

[10thTx]

Fantastic! Thanks so much for sharing the schematic! Remarkable build. Appreciate your taking the time to work the schematic up for us. It looks very well thought out and documented.

with respect, 10thtx

[bepone]

thanks 10thTx,
i'm playing with the cabinets now.. external cab works fine.. at the end looks that this small amp deserve one or two sizes bigger cabinet, this small one is maybe too small for D type amp.. limiting low end (affecting also low middle) . so i will transfer the amp in bigger box (50 liter) when i will have some free time