Fate of my Heathkit W7-M amplifiers

[riotpack]

I have just purchased two Heathkit W7-M amplifiers (1X6AN8 & 2xEL34 PP) which someone has modified to use a 12au7. I was thinking about restoring them but it looks like it may take too much work, my next plan is to strip the chassis and rebuild them on a nice new chassis.
I have ordered Siemens NOS EL34s and Japanese hit-ray 6AN8s to use with this amp and will be placing an order for new caps and resistors and I will redesign the PSU with a choke this time and more filtering.
One of the output transformers has a dent in the top (minor) and I was wondering if anyone knows if I can take this top "cap" off without damaging anything? Also what are some recommendations for the fate of these amplifiers?

[HollowState]

In my opinion restoring or rebuilding onto another chassis seems like almost the same amount of work. I guess it really depends on just how bad off the mechanics of the original is regarding rust and/or corrosion. And also how much you like the original look. That's a judgment only you can make.

Regarding the dented transformer, I looked at pictures on line and have seen that the trannies are horizontal double half shells. So yes, you can unbolt the covers and fix the dent without and ill effects. Hopefully the dent hasn't mashed the coil build underneath. It appears that the transformers have a bolt between the chassis and bottom shell, so totally removing the transformer to work on it may be necessary. It the OPT bolts have an insulated shoulder washer on one end, don't discard it. It's there for a reason which is to prevent the bolt from shorting the lamination stack.

Adding a choke may reduce B+ somewhat and you may find that's it's of little benefit to performance, even though Heath did things on the cheap.

[riotpack]

I am getting prices on the Hammond HWCHAS1710BK chassis X2 (Hammond Mfg. - Walnut Chassis with Aluminum Top Panel (HWCHAS Series)) and transformer covers etc. It looks like someone has removed the variable damping selector but the amplifiers are sitting in the workshop at the moment until I get some 230-110V transformers to power them up. Regarding the change to 12au7 (or is it 12aT7), are these better at driving the EL34s than the original 6AN8 configuration?

[HollowState]

Originally Posted by riotpack

Regarding the change to 12au7 (or is it 12aT7), are these better at driving the EL34s than the original 6AN8 configuration?

EL34s are reasonably easy to drive so either tube is capable of doing the job. Of course this will depend on the circuit configuration chosen. The pentode section of the 6AN8 will provide more gain for the input stage and give the amplifier a little more input sensitivity. Some folks don't like pentodes for reasons of their own. Be it preceived sonics or topology mindset. But there's nothing wrong with pentodes. It's really a matter of designer's choice and preferences since there are literally dozens of ways to drive the output tubes.

[riotpack]

Thankyou for rhe responses, very helpful! Are there any other modifications I can make to the schematic to improve the sound as I rebuild this?

[riotpack]

Here are some pics of the amp - You can see the bridge rectifier and other mods someone has done in the past. I am buying a new enclosure and mounting everything on it and converting it back to 6AN8 for the first stages.... That OP transformer is pretty big too!

[riotpack]

Originally Posted by HollowState

EL34s are reasonably easy to drive so either tube is capable of doing the job. Of course this will depend on the circuit configuration chosen. The pentode section of the 6AN8 will provide more gain for the input stage and give the amplifier a little more input sensitivity. Some folks don't like pentodes for reasons of their own. Be it preceived sonics or topology mindset. But there's nothing wrong with pentodes. It's really a matter of designer's choice and preferences since there are literally dozens of ways to drive the output tubes.

So this may be why someone converted the 6AN8 stage to a 12AU7. This just makes it more complicated for me to decide which valves to use during the rebuild?? I have some NOS 6AN8s coming, but if 12AU7 is a better choice I can get some of them.

[riotpack]

Sorry those pics will not expand for some reason. Try here http://profile.imageshack.us/user/riot454ci/

[dcgillespie]

Converting the amplifier to a 12AU7 front end will certainly make for an all triode front end, but ultimate performance by specification will be no where near that of the original. Using an all triode front end will reduce OLG so low, that closed loop performance will suffer significantly. I would suggest that you build the original circuit so you know how it sounds when operating as designed. Then, you can experiment from there as you might wish.

Dave

[dgta]

One option is to use a 7247 tube. That will keep it all triode and have very good gain in the first stage.