loosen the poweramp

[mat]

I have a 50W 124 (built in dlator) style build that has evolved many versions to date (actually my test bench build). Latest is I built 10thTX and geezers Tweed Overdrive Specials pre amp into it. At lower volumes it is really good sounding amp, at band level it is way too tight for my playing style. Side by side with the previous builds (nonHRMskyline and 2555) it is too tight sounding.

How could I achieve more loosen power amp section? I tried Delta Pro 12A but prefer RWB on this amp. I'm afraid the PT does not handle tube rectifier. The build has 5 novals and two octals. Overall voltage drop ?

Any tips ?

Thanks,

[Tonegeek]

I have a 50W 124 (built in dlator) style build that has evolved many versions to date (actually my test bench build). Latest is I built 10thTX and geezers Tweed Overdrive Specials pre amp into it. At lower volumes it is really good sounding amp, at band level it is way too tight for my playing style. Side by side with the previous builds (nonHRMskyline and 2555) it is too tight sounding.

How could I achieve more loosen power amp section? I tried Delta Pro 12A but prefer RWB on this amp. I'm afraid the PT does not handle tube rectifier. The build has 5 novals and two octals. Overall voltage drop ?

Any tips ?

Thanks,

Try a 330R sag resistor on the supply instead of the choke. Lower filtering is supposed to loosen it up too. #124 has 150uF on the main caps. Going to ~110uF may help. I like it in the non-HRM I just built.

It could be the preamp. (you have 100k plates right?. If not, try that sometime) What's your voltages on V1, V2? It will get more compressed if you lower the voltages on V1. Tone will change if you go too low. You could just try some different tubes as voltage could vary a lot depending on the tube. Otherwise use 150k to 220k (1-2 watt)resistor to ground from the last filter node (FET load simulator) to get the voltage down to 190 (or lower if the tone is still OK - depends on the tube.) Should get some looseness now. If the voltage is more than 220 on V1 you may have to play with the dropping string in the PS to get it low enough. Do you have a schematic of the preamp you could share. Sounds interesting.

[Structo]

Yes but a 50 watt amp should only have around 50uF on the plates.
Two 100uF in series. You could try two 80uF in series to get 40uF.

[John_P_WI]

Mat there are many ways to loosen the power amp. Some of the better known methods that work are:

Change the negative feedback, easy to adjust, put a 50k pot in series with the feedback off of the speaker tap. This has been known as a "soul" control in the '90's.

Change the negative feedback by changing which tap it is fed from. Example, switch from 16 ohm to 8 ohm tap. Easily switchable.

Put a power resistor in series from the filter cap node to the center tap of the output transformer. 100 ohm or larger, calculate the power rating needed. This will add sag to the power amp only. Of course the larger ohm resistor the larger the sag. Put a switch across the resistor to switch it in and out if needed. Season to taste.

Put a small resistance between the cathode of the filter cap(s) and ground of the output transformer center tap filter node. 10 ohms or so... This simulates ESR of old caps.

Cathode Bias the amp, don't bypass the cathode resistor with a cap, or make it switchable with a cap. Too much bass or too stiff with a cap? Add resistance to the cap bypassing the resistor.

BTW, KOC from London Power / Ultimate tone has variable sag circuits too, an extension of his power scaling kits.

Just a few quick thoughts, hope they help.

John

[mat]

Hey, big thanks for the suggestions. I'll start trying them one at time.

[Tonegeek]

Yes but a 50 watt amp should only have around 50uF on the plates.
Two 100uF in series. You could try two 80uF in series to get 40uF.

Right on. I missed that (was going by the #124 schematic which is a 100 watter). I got 100 watts on the brain since I been doing Twin tweakage lately

[mat]

Change the negative feedback, easy to adjust, put a 50k pot in series with the feedback off of the speaker tap. This has been known as a "soul" control in the '90's.

Done. A 50k pot after the fb resistor going to 4ohm tap.

Put a power resistor in series from the filter cap node to the center tap of the output transformer. 100 ohm or larger, calculate the power rating needed. This will add sag to the power amp only. Of course the larger ohm resistor the larger the sag. Put a switch across the resistor to switch it in and out if needed. Season to taste

.

Done. I only had a 470R 10W resistor. I'll try with that first (it's on a big switch).

Put a small resistance between the cathode of the filter cap(s) and ground of the output transformer center tap filter node. 10 ohms or so... This simulates ESR of old caps

.

Sorry, I don't know what You mean ?

Cathode Bias the amp, don't bypass the cathode resistor with a cap, or make it switchable with a cap. Too much bass or too stiff with a cap? Add resistance to the cap bypassing the resistor.

Cathode biasing the amp is something that I was thinking about. 10thTX also suggested that and his amp sounds great.

BTW, KOC from London Power / Ultimate tone has variable sag circuits too, an extension of his power scaling kits.

I'll try to google that.

Just a few quick thoughts, hope they help.

I'm sure they will. Bad thing its past 11:00pm here. Cannot fire the amp but tomorrow.

[John_P_WI]

Put a small resistance between the cathode of the filter cap(s) and ground of the output transformer center tap filter node. 10 ohms or so... This simulates ESR of old caps

.

>Sorry, I don't know what You mean ?<

Funny re reading my post I see how confusing it is to myself.

What I meant: The first resevoir cap (Node A) I assume feeds the center tap of the output transformer.

This cap(s) Node A, 47u, the cathode is connected to ground. It is possible to place a small resistance between the cap(s) cathode and ground. This simulates the resistance of the older less efficient caps of many years ago.

ESR, think of it as Electrical Series Resistance of the cap.

The addition of a small resistance to new caps adds a little vintage vibe, start at 10 ohms and move higher if needed. Of course the resistor can be "shunted" with a switch to add or remove it from the circuit.

John

[mat]

It could be the preamp. (you have 100k plates right?. If not, try that sometime) What's your voltages on V1, V2? It will get more compressed if you lower the voltages on V1. Tone will change if you go too low. You could just try some different tubes as voltage could vary a lot depending on the tube. Otherwise use 150k to 220k (1-2 watt)resistor to ground from the last filter node (FET load simulator) to get the voltage down to 190 (or lower if the tone is still OK - depends on the tube.) Should get some looseness now. If the voltage is more than 220 on V1 you may have to play with the dropping string in the PS to get it low enough. Do you have a schematic of the preamp you could share. Sounds interesting.

Hi Tonegeek. The preamp is from 10thTX's and Geezers 'Tweed Overdrive Special' amp. I might tweak the preamp later but I have a feeling that tweaks to the power amp will make the amp better.

[John_P_WI]

Another fun thing to do, which is great to add touch response to a hard amp, is to change the 470 ohm screen resistors on the output tubes to say 2200 ohm or even higher.

I don't build anything with less than 1K ohm in the tube position here, helps with the tube life too.

2k2 and above will add some squish. I have used it to add touch response to cold Marshall circuits before.

[Tonegeek]

It could be the preamp. (you have 100k plates right?. If not, try that sometime) What's your voltages on V1, V2? It will get more compressed if you lower the voltages on V1. Tone will change if you go too low. You could just try some different tubes as voltage could vary a lot depending on the tube. Otherwise use 150k to 220k (1-2 watt)resistor to ground from the last filter node (FET load simulator) to get the voltage down to 190 (or lower if the tone is still OK - depends on the tube.) Should get some looseness now. If the voltage is more than 220 on V1 you may have to play with the dropping string in the PS to get it low enough. Do you have a schematic of the preamp you could share. Sounds interesting.

Hi Tonegeek. The preamp is from 10thTX's and Geezers 'Tweed Overdrive Special' amp. I might tweak the preamp later but I have a feeling that tweaks to the power amp will make the amp better.

After I posted I realized my suggestions would probably only be good for the Dumble style preamp, so scratch my suggestion on that. I am sure you will get it working right with some of the other suggested tweaks. I have heard suggestions before about upping the screen resistors. Might try that one myself sometime.

[mat]

Another fun thing to do, which is great to add touch response to a hard amp, is to change the 470 ohm screen resistors on the output tubes to say 2200 ohm or even higher.

I don't build anything with less than 1K ohm in the tube position here, helps with the tube life too.

2k2 and above will add some squish. I have used it to add touch response to cold Marshall circuits before.

Great, I'll try 2k2 screens also. Btw. I want to be sure if I got it right on this time. Is the pic as You mentioned the mod to be:

http://mattitee.kuvat.fi/kuvat/temp/PSmod.jpg

[John_P_WI]

Yes, the pic is very similar except the pic is on the screen node. I was suggesting the B+1 node. You can try either or both and decide which node(s) add the effect you like.

The resistor shown can be bypassed by a switch for a vintage / modern feel.

[mat]

Yes, the pic is very similar except the pic is on the screen node. I was suggesting the B+1 node. You can try either or both and decide which node(s) add the effect you like.

The resistor shown can be bypassed by a switch for a vintage / modern feel.

Ok, I'll experiment some more. I tried the amp today with neighbour friendly volume and I like what the 470R does on the OT center tap. I'll try it at the weekend with proper volumes.
I noticed that the JJ6V6S plate voltages were 20V lower when the 470R was activated. Does that seem to be right ? I biased the 6V6's about 12W and the result was quite better than at 10W. Plates were at 467V with the 470R and 487V without.

The 50k pot at 4ohm tap gave lots of very bright sounds but I preferred the pot off option. Will check that too with proper volumes.

I'm hunting the 2k2 5W for the grids. Will report how the progress goes.

Thanks,

[John_P_WI]

Yes, the voltage drop is to be expected. It is fun if you have a fast meter to measure the volt drop across the resistor on the center tap and hit a power cord.... Truly demonstrates sag.

With the feedback and "soul" pot, you can always change back to the 8 or 16 ohm tap to find which you like best.