Amp kit build help

[ryanuk]

Hiya

I'm in the middle of a amp build from a kit I purchased from musikding The design is based upon 2 el84s in push pull, dual channel, 15w.

I've assembled the amp but am having some issues; the symptoms are;
1. The amp only the expected sound with 1 of the power tubes inserted. With both tubes inserted it goes terribly gated and distorted.
2. Heater voltages are too high at > 7v. I think this is due to it being a kit designed for "EU" voltage at 220-230v, and UK voltage is @ 240v. I measured it at 242v.
3. There is a low level hum, possibly due to heater voltage? Or perhaps lead dress.

I've eliminated the tubes as a problem as I've had some test these in another amp.

The first problem I suspect maybe wiring error at the PT. Im checking and double everything but wondered if anyone had any other ideas for symptom 1?

[eschertron]

The amp only the expected sound with 1 of the power tubes inserted. With both tubes inserted it goes terribly gated and distorted.

Does it matter which tube? Does it matter which socket? If only one socket works, I'd suspect a wiring error on the second socket. The other wiring error that comes to mind is the possibility that the signal at the two power tube grids are in phase, cancelling out when combined. Can you provide a schematic? Some gut shots? That will help with any suggestions other than "may be".

Oh, and welcome! You will get lots of helpful comments from members here

[The Dude]

...... Can you provide a schematic? Some gut shots? That will help with any suggestions other than "may be"......

+1

It will help us immensely if we can see what you built.

[mort]

There's always a 50/50 shot at getting the OT primary windings reversed.

[ryanuk]

There's always a 50/50 shot at getting the OT primary windings reversed.

Many thanks - will take a look at this

[ryanuk]

Hopefully these should help. I dont have a pic of my wiring yet. Will take some pics asap.

[ryanuk]

There's always a 50/50 shot at getting the OT primary windings reversed.

Can I ask a question. Attached is a schematic of my OT.
Do you mean getting A1 and A2 confused?
Or A1/A2 with the B+?

[DrGonz78]

A1 goes to one plate of one of the power tubes and A2 goes to the other power tube's plate. Simply reversing these connections is all that is being recommended. Although I know this really matters mostly in cases where there is negative feedback in the circuit, but looking at the schematic there is no negative feedback loop. Perhaps though reversing these connections might fix your problem. Can't hurt to try it out really.

[ryanuk]

Hi
Update to my issue.
I triple checked my wiring against the schematic and layout plan. I did some rework in places where I thought I may have dry joints and the busy phase invertor wiring with all new components. No change to the problem. Sound is still gated and horrible.

I rewired the OT, and tried swapping the plate wires round. No change to the problem. Sound is still gated and horrible.

I then noticed one of the (multiple) secondary OT wires hanging by a thread! It inspected it and it broke in my hand. Ive come to the conclusion that this connection was already broken and only held in place by the potting compound. Im appealing to the website to provide me with a new OT, or the cost of one. Sound is still gated and horrible after testing with the broken OT.

Whilst I source a new OT, I have a few more questions re some of the amp voltages.

All the amp voltages at the test points, are too high and just out of recommended tolerance. I believe this is because the kit is built for a 220v AC wall voltage, and in the UK we have 240v.
Ive used Ohms law and sourced a pair of 2W resistors to drop the heaters down from 7.4VAC to the recommended 6.6VAC.
However, because the B+ is higher, so is the the resulting bias voltage (10.7V - the manual says not to exceed 10.5 as I assume this will take me above the recommended plate dissipation);

1. Is this amp Cathode Biased? Im assuming yes, and the Bias is determined by R28 and R29 in the schematic I provided.
2. Should I bring the overall B+ down by adjusting R33, or shall I just adjust the bias of the tubes by adjusting R28 and R29? The sensible approach seems to be adjust R33 as this will bring all the voltages down proportionally?

Can anyone help?

RyanUK

[mozwell]

First, im no expert, but here is what i would do
Do you have an audio probe (google it), you can probe the gate of each valve to see where the signal is getting "gated & horrible"

Yes, this is a cathode biased amp, you could try increasing R28 & R29, try 390 ohm as this will run the output tubes "less hard" & you can then see if this fixes your issue.

What are you using for a sound source, a guitar, or say an MP3 player output. If you are using a guitar, turn the guitar volume pot to half or a quarter, does the amp still do this ?
Does the amp still sound horrible on both clean & crunch modes,

The amp must always be run with a load. If you like when using your audio probe, pull the two output valves, so you can probe the preamp.

With no SIGNAL on the input, and the input shorted, please post DC voltages with respect to 0V for each preamp valve & the phase inverter valve
eg V1B pin 1 +123VDC, pin 2 0VDC, pin 3 +1.20VDC

Please measure the heater AC voltage directly at the socket of each valve. Measure from pin 9 to pin 4 (or pin 5) of each preamp & phase inverter valve
As you are using 6.6V heaters, pin 4 & 5 should be tied together for each preamp & phase inverter valve
Power amp heater is 6.6VAC across pin 4 & pin 5

Once we have verified all power supplies & heaters are ok, we can move forward

Also, please double check the value of all resistors & that they correspond to the schematic. It may just be you have one wrong resistor leading to a large drive level to one valve. Also double check all your wiring as a precaution.

Photos of your build will help the community here, there may be something obvious that one of us here can find for you

[Dave H]

1. Is this amp Cathode Biased? Im assuming yes, and the Bias is determined by R28 and R29 in the schematic I provided.
2. Should I bring the overall B+ down by adjusting R33, or shall I just adjust the bias of the tubes by adjusting R28 and R29? The sensible approach seems to be adjust R33 as this will bring all the voltages down proportionally?

The output stage is biased with a common cathode rsistor so it shouldn't be operated with only one EL84.

1. Yes and Yes.
2. Except for the heaters I wouldn't bother to bring the voltages down as the amp has a lower B+ voltage than is normally used in these amplifiers. My amp has the same 135 ohm cathode resistor and measures B+ 336V, EL84 Vk 11V and has done for over 10 years.