[dude_500]

I built dsavitsk's 6-tube amp (ecp.cc) about a year ago and it has been sounding great ever since. Recently, however, someone has asked me to record some records for them which I started doing yesterday. The recordings sound great, however since this person will be checking the waveforms for purity, I began to notice something odd in my own inspections. There doesn't appear to be any DC offset, however the actual wave form tends to be heavy on the negative side of neutral than it does on the positive. I have moved all the tubes around so that doesn't seem to be the problem unless all of mine are damaged. Is this normal for vinyl or is there something that I need to change in the amp?

The problem becomes more significant in loud sections of music. This part isn't particularly significant but it's the record that I had on so it was easiest to grab and the problem can be illustrated with it.

For reference, I am running the output through my beta-22 for volume control then recording through an M-Audio Audiophile 192 card.

 

[funch]

I'm wondering if it could be related to 'table rumble.

 

[dude_500]

Quote:

Originally Posted by funch /img/forum/go_quote.gif I'm wondering if it could be related to 'table rumble.

Unless there's something inaudible that would result from that I don't think that's the problem. It's dead silent with the needle up and spinning and during lead in/out I hear no rumble at all.

 

[dsavitsk]

Looks like it might be clipping. Where are the tube's biased? Try building something like this Hagerman Technology LLC: iRIAA Filter for Phonostage DIY Kit and then running it through RMAA

 

[dude_500]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif Looks like it might be clipping. Where are the tube's biased? Try building something like this Hagerman Technology LLC: iRIAA Filter for Phonostage DIY Kit and then running it through RMAA

It is built exactly by the schematic on your site.

I'd rather not need to wait to get parts and complete another little project since I want to get on this recording, but if need be I'll build one of those little RIAA boards.

 

[dsavitsk]

Quote:

Originally Posted by dude_500 /img/forum/go_quote.gif It is built exactly by the schematic on your site.

Doesn't mean everything is working as it should.

Also could simply be over driving something if you don't need all the gain.

Quote:

Originally Posted by dude_500 /img/forum/go_quote.gif I'd rather not need to wait to get parts and complete another little project since I want to get on this recording, but if need be I'll build one of those little RIAA boards.

You are probably going to need to send some test signals through it one way or another. If you don't have a test record, then you'll need to build an inverse RIAA curve circuit so you can use RMAA or a test CD or whatever. A scope would be a good thing, too.

Additionally, measure the bias of the cathodes and measure the plate voltages of all the tubes.

It is also possible that your record player is not working right, so if you have access to a second record player, that might be illuminating.

 

[amb]

I am not sure if there is a problem. Music signals are not necessarily symmetrical above and below the "zero" crossing line, so what you see may in fact be what it should be. To really determine whether your system is "doing something it should not be doing" to the signal, use more predictable test signals such as sine, triangle and square waves instead.

 

[dude_500]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif Doesn't mean everything is working as it should. Also could simply be over driving something if you don't need all the gain. You are probably going to need to send some test signals through it one way or another. If you don't have a test record, then you'll need to build an inverse RIAA curve circuit so you can use RMAA or a test CD or whatever. A scope would be a good thing, too. Additionally, measure the bias of the cathodes and measure the plate voltages of all the tubes. It is also possible that your record player is not working right, so if you have access to a second record player, that might be illuminating.

I tried it on an old stereo receiver and the signal is roughly symmetrical, at least a lot more than it is through this amp.

The plate voltages of the 6GK5's are 155 when cold and (115, 117, 114, 107) when hot. Plate voltages of the 6T4's are 104 and 107 both hot and cold (few volts variation hot)

I'm assuming measuring the bias means measuring the voltage at the grid which is: For input 6GK5's: 50mV cold -25mV hot, for the next 6GK5's: 100mV cold -175mV hot, for 6T4 followers: 31V cold to 33.5V hot

I'm not sure if these values are good or bad, does this seem right?


I'm going to go generate some waveforms into it and look at them on a scope to see if it really is deficient in the positive half of the range

 

[dsavitsk]

Quote:

Originally Posted by dude_500 /img/forum/go_quote.gif I'm assuming measuring the bias means measuring the voltage at the grid which is: For input 6GK5's: 50mV cold -25mV hot, for the next 6GK5's: 100mV cold -175mV hot

Grids should be at ground. If they really are at -25mV and -175mV, that suggests that you are drawing grid current. And, the numbers are consistent with this. Probably not ideal. Might indicate that some rebiasing and tinkering is in order. This is probably not your issue here as these tubes should be pretty linear even with some grid current -- basically a little grid leak biasing going on. I'll double check mine to see if mine does this, though my recollection is that it does not.

But, you really want to measure from the cathode to ground -- across the bias setting resistor.

 

[dude_500]

Cathode voltages are (622mV, 627mV) input tube, (662mV, 666mV) second tube, and (36.2V, 35.7V) for the 6T4 follower

I hooked up a scope and did indeed see a problem with the top of the voltage range at higher input levels. Input settings are in millivolts range output in volts of course.

 

[dsavitsk]

What is your B+? If you have the voltage, one thing you might want to try is increasing the bias resistors and decreasing the stage output resistors in turn. For instance, the first stage has an output impedance of rp + mu*Rk + 51K which is around 61K (this works in conjunction with the RIAA filter, so you want this to stay the same). So, increase the 56 ohm resistor to 150 ohms and decrease the 51K to around 43500. This should stop any grid current from being drawn and give you plenty of bias. You can do something similar for the second stage. Only real issue is that you'll need around 200V of B+. Mine is biased low because I am using a line isolation transformer and only have about 150VDC to work with.

If, however, the clipping is due to too low a plate voltage, then this fix is exactly backwards. FWIW, the 6GK5's do seem to have a tenancy to run a little hot.

 

[dude_500]

B+ is 155V but drops down to around 110V when it warms up.

I could setup a 200V supply but it's by no means my preferred method since in the long term I'd have to get another transformer and upgrade my caps... I'm also running on an isolation transformer.

 

[dude_500]

What part(s) do I adjust to lower the gain? I don't need all the power this amp gives and it seems like the problem is when there are loud sections.

 

[nikongod]

In a tube amp with plate CCS's you use different tubes to adjust the gain. This (typically) requires that the CCS current be adjusted to suit the new tube. After that, since the ouptut impedance of the tube has significant effects on the actual frequencies where the EQ works you adjust various things in the EQ circuit too. Really you just redesign the amp if you want less gain.
Quote:

Originally Posted by dude_500 /img/forum/go_quote.gif B+ is 155V but drops down to around 110V when it warms up.

B+ is not 155V, 155V is the maximum voltage between when the amp is turned on, and the amp stabilizes. B+ in your amp is 110V.

The plate voltages you posted above show only 10V at best across plate CCS's. that is DANGEROUSLY close to the cutoff points for a SS CCS, especially considering the deceptively large voltage swings that a phono-stage may need to make.

The second output waveform you posted shows very obvious asymmetrical clipping. AKA, the CCS's dont have enough voltage overhead to do their thing.

They are not the bible, but several posts ago (while debugging this when originally built) the 2 morgan jones books were recommended. were they obtained and studied? It sounds like you tried to copy a design and make changes on the fly without understanding the changes or reasoning behind the original choices. There is nothing wrong with making your own modifications to someone elses design, but that is assuming you know how what you are changing will effect the circuit at the end of the day. PLEASEEEEE! get them.

 

[dude_500]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif In a tube amp with plate CCS's you use different tubes to adjust the gain. This (typically) requires that the CCS current be adjusted to suit the new tube. After that, since the ouptut impedance of the tube has significant effects on the actual frequencies where the EQ works you adjust various things in the EQ circuit too. Really you just redesign the amp if you want less gain. B+ is not 155V, 155V is the maximum voltage between when the amp is turned on, and the amp stabilizes. B+ in your amp is 110V. The plate voltages you posted above show only 10V at best across plate CCS's. that is DANGEROUSLY close to the cutoff points for a SS CCS, especially considering the deceptively large voltage swings that a phono-stage may need to make. The second output waveform you posted shows very obvious asymmetrical clipping. AKA, the CCS's dont have enough voltage overhead to do their thing. They are not the bible, but several posts ago (while debugging this when originally built) the 2 morgan jones books were recommended. were they obtained and studied? It sounds like you tried to copy a design and make changes on the fly without understanding the changes or reasoning behind the original choices. There is nothing wrong with making your own modifications to someone elses design, but that is assuming you know how what you are changing will effect the circuit at the end of the day. PLEASEEEEE! get them.

I don't have those particular books but I do have a number of other tube reference books as well as some old ARRL Handbooks which have excellent tube content in them. I've recently begun to study this more but don't yet have the expertise to fix this problem. I'll keep reading about it and hopefully be able to fix it sometime soon. I figured I'd post here hoping it was an easy fix but it looks like it's not.

How do you get the 10V CCS overhead? It seems to me the CCS's are taking about 40V if it drops from 155 to 115 volts. Is there an easy way to increase this overhead voltage?