The Amp Garage

So final posting on this preamp: So the questions about the LNFB is somewhat solved for me. I tried the LNFB from either V2 plate to V2 grid (as in the schematic by Bombacaototal) or from the cathode of the CF after V2. Almost no real difference. I recorded the two different LNFB variants using a lo...

Update: I now have a 18pF across the 3M3 and this gives some nice clarity (not that the sound was muffeled before, just a bit more sparkel now). I am still wondering about the Local Negative Feedback around V2. Max ("just the facts") and somebody called Jay stated many years ago, that the LNFB on V2...

So again on the V2 low plate voltage topic: I bypassed the 0.1 screen capacitor (Cs in pic below) with either a 1M, 820k or 680k resistor and this brought the plate voltage up and the screen voltage down. In case of the 680k I had about 85V on the plate and 83V on the screen. But the sound changed q...

Hi Bepone, Thank you for your reply. I managed to fix the 100hz noise. It was ripple from the 6.3V DC supply for the EF86s. I used a 7805 with two diodes between the 7805 ground and circuit ground to boost the voltage to 5.7V and that posed some problems. I changed the 7805 to a 7806 (without diodes...

Hi Rootz, Thank you for your tip. I will try that. The values are not really verified of any of the components. But I take them to be correct for the time being. Maybe the Screen voltage node was different than for V2 anode in the real amp? With a lower screen voltage supply, the anode voltage would...

Swapping the EF86 did indeed result in slightly different readings: V2 (swapped EF86, now Telefunken, was Valvo): Anode: 41V (was 25V with Valvo) Screen: 120V (was 100V) Cathode: 2.0V (was 2.2V) V1 (now Valvo, was Telefunken): Anode: 161V (was 191V with Telefunken) Cathode: 3.6V (was 3.3V) So still ...

Looking at the AC15 schematic using EF86, the anode resistor is 220k, cathode resistor 2k2 with 25uF, so comparable, screen resistor 1M. Still in the AC15 you have 90V on the anode with 315V on the string. https://www.ampbooks.com/mobile/classic-circuits/vox-ac15/ Could it be the tube? I will swap t...

Here are some voltages and a head scratcher: V1: Anode: 191V, Cathode: 3.3V V2: Anode: 25V, Cathode: 2.2V (that is the second pentode with the 240k anode resistor) V3a (CF): grid: 25V, Anode: 316V, Cathode: 39V V3b: Anode: 210V, Cathode: 1.7V PS String Voltages: V1: 296V V2 and V3b(CF): 318V V3a: 32...

Schematic of preamp and tremolo attached. I introduced a 220k grid leak resistor to ground on the last triode. I tried to at least stay somewhat close to the layout of the original amp, albeit I have the tubes mounted behind the boards, not under the boards. Attached the layout of the original. cour...

Hi gang, Based on the schematic provided by forum member Bombacaototal I set out to pay homage to the late Mr. Dumble: The Pentode Amp (aka Silver Alligator) as a stand-alone preamp version. With Tremolo but sans Reverb (I have some ideas for this using a stand-alone studio grade Spring Reverb unit ...

Thanks for the kind words guys! @Ryan: the pt which has 270v ac (80ma) and 14v ac (2A) is from a german parts supplier. 49€. https://www.musikding.de/Toroidal-transformer-230V-270V-14V I also ordered a custom wound 290v, 14v, 6,3v which has not arrived yet, but the musikding pt works perfect with 3 ...

Fellow Ampgaragists! After a long hiatus, the Dumble bug bit me again and I had a spare 19" 1 Unit enclosure lying around since 2015. I also sold all my Dumble ODS amps over the last few years, since I did not really use them anymore. Still I missed the Dumble ODS sound and the one remaining clone I...

Dear All, so I recently acquired this nice little Orange OR15 amp which sounds quite nice I must say. The only problem with the amp is that it has a nasty 50hz heater hum. I redressed and twisted the heater wires and also installed an elevated heater mod, which helped some, but the amp still has som...

martin manning wrote: ↑Wed May 27, 2020 4:37 pm Interesting. No question you don't need the higher anode voltage. Did you happen to measure the cathode voltage when on standby?

Hi Martin,

I forgot to measure the cathodes while on standby. I will do that the next couple of days, I put the chassis back in the cabinet already.

Electron

Update: I installed a 22k (2W) resistor in series between B+ and the CF driver anodes and dropped the voltage to 380V (B+: 435V), this calculates to 2.5mA for the CF driver. I measured the bias and the output tubes lost 1mA. I do not hear much of a difference between the high CF driver voltage and t...