Dynaco Mark III Rebuild: low output problem

I recently rebuilt a Dynaco Mark III with help from everybody on this site a few months ago. I didn't get a preamp to test it until recently.

Now that I have one (a 25 ART professional tube mic preamp) I'm getting very nice sound in low volume up until I push it too hard and then I'm getting a distorted clipping, farting type noise.

I can't seem to get a good loudness that I would expect from a power amp rated at this many watts without the ugly breakup. All the voltages check out and everything has been rebuilt.

The speaker is tested and goes wicked loud on my solid state stereo and is 8ohms. I think there might be something sucking output to ground as there arent many ohms from the outputs to ground?

Any ideas?

I'm not familiar with the 25 ART, but I believe those are MICROPHONE preamps. If you are feeding a LINE LEVEL signal such as from a CD player, digital audio source or FM tuner into a MICROPHONE input, you will overload it to clipping in short order. If that preamp can be switched from Microphone to Line level, make sure you have done that.

Well the clipping isn't the problem I have. It's the overall volume being...disappointing. I'm not getting the clipping until the preamp is boosting nearly 40 db...

Should this power amp deliver a lot of volume? or is there something wrong with it itself? I mean a 60 watt tube amp for guitar would blow the walls apart with sound.

steve's garage wrote:I recently rebuilt a Dynaco Mark III with help from everybody on this site a few months ago. I didn't get a preamp to test it until recently.

Now that I have one (a 25 ART professional tube mic preamp) I'm getting very nice sound in low volume up until I push it too hard and then I'm getting a distorted clipping, farting type noise.

I can't seem to get a good loudness that I would expect from a power amp rated at this many watts without the ugly breakup. All the voltages check out and everything has been rebuilt.

The speaker is tested and goes wicked loud on my solid state stereo and is 8ohms. I think there might be something sucking output to ground as there arent many ohms from the outputs to ground?

Any ideas?

When connected to a dummy load , how many watts will it deliver when it starts to clip ? And what is the voltage on input then ?
Measurement is the only way to figure out if the amp follows spec.

And how efficient are your speakers?

You haven't mentioned what source you are testing with.
I suspect that your problem here is the preamp. I have an ART mic pre in my recording facility and it is only useful with a low impedence microphone-level (extremely low condenser mic type) signal. If you are feeding a line-level into the ART you're dealing with not only a level mismatch but also a gross impedance mismatch.
If you have a line-level source to test with (CD player, etc.) you can plug it directly into the input of the MKIII, although without any attenuation you are likely to get blasted with full volume. But you'll know right away if, as I suspect, the ART pre is the culprit.

Dollars to donuts it's the preamp. Levels are mismatched so your input is overdriving the preamp and causing distortion and saturation before it even hits your Mark III. Like plugging a receiver or line level device into the phono (mic level) input of a preamp. When you get this straight and everything in balance you will likely have a monster sound.

Do you have an IPOD that you could connect directly to the amp? The IPOD is line level with a volume control.

Peterh: To get the wattage measurement I assume I should measure across the dummy resistor and calculate the math for watts? And for the input its just dc volts across the input jack?

Everyone else: I've got the donuts if you've got the dollars. A straight ipod in gives me very low, albeit clear sound. Boosting an Ipod or phone in gets my volume up until after something like a 40db boost on the preamp where I start to clip in the power amp.

I don't know the speaker efficiency but they are capable of giving me AT LEAST 1.5 times the volume through my solid state amplifier with ease.

I'm suspecting a ground leak in the input section at this point.

steve's garage wrote:Peterh: To get the wattage measurement I assume I should measure across the dummy resistor and calculate the math for watts? And for the input its just dc volts across the input jack?

Everyone else: I've got the donuts if you've got the dollars. A straight ipod in gives me very low, albeit clear sound. Boosting an Ipod or phone in gets my volume up until after something like a 40db boost on the preamp where I start to clip in the power amp.

I don't know the speaker efficiency but they are capable of giving me AT LEAST 1.5 times the volume through my solid state amplifier with ease.

I'm suspecting a ground leak in the input section at this point.

Measure the voltage across the dummy load, connect a sinus generator with variable output to the input. Observe the signal across the dummy load while increasing the generator until clipping is observed. Measure the voltage on the dummy and on the input when the signal is just under clipping.
Power generated is calculated by the formula P= V^2 / R
or in text, power in watts = voltage ( mean not top ) squared volt divided by dummy load ohms

I think that less then 2 V on input is needed to generate 60w power. I have no MkIII on hand and the manual gives no number. Anyone with a figure ?

.

Do you have a good 6AN8? It might be beneficial to clean the pins on the tubes and retention the sockets.

Cleaned and retensioned the preamp socket. I will check the input and output readings tomorrow.

The preamp input tube socket is put to original...Is there any problem with putting the preamp before the input jack and just leaving the strange preamp tube socket in with no tube or connector connected to it?

practically guaranteed you are overdriving the preamp, it's not made to handle line-level sources