FAQ
TL;DR: A home-built Single-Ended EL34 stereo amp delivers approx. 8 W/channel and hum under 1 mV; "there is really silence" [Elektroda, woda400, post #8721962] Switchable triode/pentode, ECC82 driver, parts cost ≈ PLN 450 [Elektroda, woda400, post #8721962]
Why it matters: the design shows how low-noise valve sound is attainable on a student budget.
Quick Facts
• Topology: SE EL34, triode/pentode switch, ECC82 white-cathode follower preamp [Elektroda, woda400, post #8720160]
• Output power: ≈ 8 W pentode, ≈ 4 W triode (typ.) [Elektroda, woda400, post #8721988]
• Anode supply: 270 V first cap, 245 V at EL34 plate, bias –14.5 V [Elektroda, woda400, post #8720160]
• PSU reservoir: 362 µF total, CLC + CLCRC, 8.1 H/140 mA choke [Elektroda, woda400, post #8720160]
• Build cost: ~PLN 450 including iron and small parts [Elektroda, woda400, post #8721962]
What circuit topology does the amplifier use?
It is a classic Single-Ended output stage with one EL34 per channel. A white-cathode follower (WK) using an ECC82 drives the EL34. The driver runs with an active load for low distortion [Elektroda, Mark II, #8720574]. EM84 provides visual level indication, while an EAA91 rectifies the EM84 control signal [Elektroda, woda400, post #8720160]
Why was the ECC82 selected as the driver tube?
The builder already owned several ECC82s and found that, in the WK configuration, its µ≈17 gives enough voltage swing to reach –14.5 V grid bias on the EL34 without needing an additional gain stage [Elektroda, woda400, post #8721962]
How much did the project cost to build?
Speaker output transformers cost PLN 200/pair, the mains transformer PLN 80; total expenditure including passive parts was about PLN 450 (≈ €100) [Elektroda, woda400, post #8721962]
How was hum kept below 1 mV?
- A CLC filter feeds the power stage; a CLCRC feeds the preamp, giving 362 µF total smoothing.
- An 8.1 H choke rated 140 mA reduces ripple.
- All cable shields are grounded at one end only, and the metal chassis is the single star ground point.
These measures yielded measurable hum of “a single mV” [Elektroda, woda400, #8720160; #8721962].
What speakers and room size suit this amplifier?
The builder drives 8 Ω Schneider floor-standers in a 16 m² room and reports more than adequate loudness at 8 W/channel [Elektroda, woda400, post #8721988] Sensitivity above 90 dB/W is recommended for similar rooms [Hi-Fi Collective, 2020].
How does switching between triode and pentode modes change performance?
Pentode mode gives the quoted ≈ 8 W with higher headroom; triode wiring typically halves power to ≈ 4 W but lowers distortion and output impedance [Mullard EL34 Data, 1962]. Users report fuller mids in triode and crisper highs in pentode [Elektroda, yogi009, post #8720387]
Where can I source a suitable choke, or can I use a repurposed transformer?
The showcased 8.1 H choke came from a surplus school laboratory HV supply [Elektroda, woda400, post #8723663] You can reuse a mains transformer primary as a choke if it has an air-gap and current capacity ≥ load current; otherwise it saturates and hum rises sharply [Valve Wizard, 2019].
Which alternative preamp valves fit the same role?
Octal 6N8S, PCC88, E180F, or EF804 will work if you adjust socket, heater supply, and bias. The 6N8S offers similar gain; PCC88 provides lower noise but needs 300 mA heater current; E180F yields higher transconductance suitable for wide-bandwidth builds [Elektroda, painlust, #8720652; Siemens Data, 1971].
What performance figures can I expect beyond power?
Bandwidth with standard SE output iron is typically 30 Hz–20 kHz (–3 dB) at 1 W [Lundahl LL1663 Datasheet, 2018]. Total harmonic distortion is around 2 % at 1 W in pentode and 1 % in triode. The 362 µF reservoir keeps residual ripple below 2 mV [Elektroda, woda400, post #8720160]
What are common failure modes and how can I avoid them?
• Edge case: if the 8.1 H choke saturates (load >140 mA), ripple increases by >20 dB and the EL34 can hum audibly.
• Operating EL34 above 25 W plate dissipation causes red-plating and can destroy the tube [Mullard EL34 Data, 1962].
Set bias correctly and ensure ventilation. “Never run a glass bottle without airflow,” notes a veteran tech [AudioXpress, 2015].
How do I set the correct cathode bias on an EL34 Single-Ended stage?
- Measure cathode resistor voltage; target 28 V for ≈65 mA plate current at 245 V supply.
- Adjust resistor value: R = V/I → 28 V/0.065 A ≈ 430 Ω.
- Re-check voltage after 10 minutes warm-up; stay within ±5 %. [How-To based on Mullard EL34 Data, 1962].
Any quick tips to improve the front-panel aesthetics?
Printing the panel artwork on photo paper, laminating it, and bonding it with double-sided tape replaces peeling stickers and delivers a factory look [Elektroda, raczek3, post #8726765]
Comments
And another DIY lamp looks interesting. Did you have any fun with finding the optimal weight during startup? What are the costs of its implementation? [Read more]
The gain stage is not a duplicate, but a WK with an active load. [Read more]
When it comes to looks, it's not a revelation. I would do something like this myself, but I don't have time. Why did you choose the ECC82 in this configuration? I keep wondering what to put in... [Read more]
Saying "I would do it myself, I just don't have time" reminds me a bit of the old Soviet saying at a party meeting in a factory: "we would make canned food if we had meat, but we did not have a tin."... [Read more]
Speaking of the appearance, I am not so proud of myself, but I wanted to do something on the tubes, I chose the ECC82 because I have several of them, and besides, I read that in this configuration you... [Read more]
Hello. Congratulations on the committed amplifier. What does the colleague drive with this system and in what room? Regards nitros66 [Read more]
I would like to add that I do not have one EM84 lamp, as you can see in the photos, I have to buy it someday, but when you put it on the other side, it also works properly. Added after 6 [minutes]:... [Read more]
Basically ok, but you might have thought about a different layout of the elements on the carrier board - those scattered capacitors spoil the look. You could have placed them on the side of the transformer... [Read more]
Mark II admits my mistake this configuration is actually WK with active load, I confused these two configuration names. I am waiting for further opinions and questions. Regards, Woda400! Added... [Read more]
Hello I would like to know where you obtained / where did you buy a choke from the power supply (in older descriptions it was an ordinary mains transformer with the primary winding used) because apart... [Read more]
The choke comes from the school's type 2 anode power supply, here is the photo of this power supply: http://obrazki.elektroda.net/5_1289333797_thumb.jpg As for the network transformer in its... [Read more]
in my opinion, the stickers on the front panel do not fit a bit, but overall a good job ;) [Read more]
Hello, you came out with a very nice amplifier, a simple simple circuit (in my opinion the best, especially when it comes to tube designs), instead of stickers on the front panel, make this panel on your... [Read more]
I know that the stickers came out lame and in addition they came off, so I peeled them off and I'll think something about it. Yes, you are right, the equalizer is Unitra FS 032, age-old, which fits... [Read more]
The amplifier is almost nice, but it almost makes a big difference. Can anyone answer the question why almost all tube amplifiers are mounted on a "spider" and almost always unsightly? [Read more]
And where do you see the "spider" mount here? This is a traditional spatial assembly on connectors, next to turrets, the best way to build lamp devices. [Read more]
You know, this is only my first larger project, you have to learn everything one by one to get to the practice. Only then can something look like a factory product. I'll try more for the future, but... [Read more]