Hello,
a couple of years ago I have bought a Dynacord DA15/V amp. The amp was in a defect state making only very weak distorted sounds.
In the last couple of years i have build a few amps and finally gained the confidence to begin with the restoration.
I would be very grateful for helps with issues i come across in the process.
Dynacord is an old German audio manufacturer. The model DA15/V was produced in the fifties. My amp was probably build in 1957 or 1958 (the speaker is dated 1957).
It uses EZ81 rectifier, two ECC83 for three separate input stages and the second gainstage, an unusual pentode/triode tube PCF82 for a third stage and cathodyne phase inverter, two EL84 for output and one more full ECC83 for the tremolo oscilator.
So far i have:
- installed a three wire mains cable
- removed the “death cap”
- replaced a cracked B+ resistor
- exchanged tubes (except the PCF82, which im yet to buy a new one)
- replaced the cathode bypass electrolytic caps (the old measured no or low capacitance, probably dried out)
- replaced all the Neokon caps (used as coupling caps and in the tremolo circuit) - not sure which material they are, but they were leaky and messing with the bias
- biased the new EL84s (470 ohms cathode resistors instead of the original 300 ohms)
I have not yet decided how to deal with the two can filter caps. They measure the right capacitance and I'm not sure if I could get and mount a possible replacement or how to mount normal caps inside the chassis where it is quite crowded already.
After these changes I get a nice sounding usable clean tone at lower volumes. I can’t really crank the amp in my current condition, so how it sounds on full volume is yet to be determined.
One thing that does not seem to sound right is the tremolo. When I switch the tremolo on, I immediately lose substantially on volume. After a couple of seconds the tremolo starts and is actually sounding very interesting and very deep at higher depth settings.
My main issue is with the large volume drop.
Unfortunately the tremolo circuit is unlike any tremolo I have seen. First, it does not use the standard three RC phase reversing circuit. Second, the oscillation does not only modulate the grid bias of output tubes the way im familiar on other amps. It also modulates the screen of the third stage preamp pentode PCF82.
I’m loosely familiar with how we handle the screen grid in output tubes - with a smallish resistor to B+ keeping the voltage a little bellow the plate voltage, but i have no idea, how it is supposed to work in this context.
Now with the tremolo off, the screen is connected through the 1M resistor to B(3)+ measuring 287V on the B+ side and 10V on the screen side. Both plate resistors from the oscillator tubes are disconnected from B+.
With the tremolo switch on, the plate resistors are connected to B+ and the screen is switched to one of the output EL84s cathode measuring 14V on the EL84 cathode side and around 1.8 on the screen side at low depth setting and oscillating between -4V and 1V on high depth setting. Furthermore B+ (3 and 4) is lowered by about 30v (it is connected to two more triodes). The connected EL84 cathode is also a little lowered and modulated.
I have found two schematics for this model. Neither of them exactly fits the circuit of my amp.
Regarding the tremolo the one attached fits, except on my amp the tremolo switch at the 1M resistor does not connect to the B(4)+ but to the B(3)+. (It connects before the second 20k resistor, not after it.)
https://elektrotanya.com/dynacord_da15v_da15w_sch.pdf/download.htmlhttps://www.radiomuseum.org/r/dynacord_da15vda_15.htmlCould anyone be able to give me a hint, what could cause the large volume drop based on the schematic and these symptoms ?
If any other information is necessary I will gladly provide it.
Thanks!
Topic: Dynacord DA15/V restoration (Read 2872 times)