HF homebrew tube amp photo build

[Radio Tech]

Well,
Got the chassis and cabinet all painted up today.
Laying out some parts to make this happen.

The photo makes the cabinet look underpainted. It is just the light reflecting off the hammertone paint

 

[Radio Tech]

Thought I would give an update.

Trying to gather parts together to see what I can use. This is mostly the slow down part of building. Several major components have to come together then the layout.

At the moment I am still in question of my plate transformer, tune and load caps.
I have some varible caps from a Pride 300 that may work. But not sure at the moment.

Was going to a hamfest this weekend but I think that is going to change. My daughter goes in the hospital this week end for minor operation. My wife will most likely be with her and also my son in law. So I may be stuck with my grandson.
Now you know I hate that

 

[linearone]

I dont know if we covered this or not but how about microwave oven transformers? they are around 2200v or so.

 

[Captain Kilowatt]

I dont know if we covered this or not but how about microwave oven transformers? they are around 2200v or so.

I'm going to try a microwave oven xmfr but in a differant application,as a filament xmfr.:blink: I was thinking that if it had a secondary voltage of somewhere around 2000 volts with 120 on the primary what would the voltage be if one were to switch the primary and secondary windings around. IOW apply 120 volts to the normal HV winding. The result was with 120 volts applied to the high voltage winding the now secondary winding had a voltage of 6.2 volts, a tad low for 572B's. I am in the process of stripping turns off of the HV winding which is now the primary and hope to get about 6.4 volts or so.A little high is easier to deal with than a little low.The other good thing is that the HV winding (primary now) has one side tied to the case and is grounded.Since I plan to run the amp off of the 240 volt line it means I can derive the 120 volts by tapping one side of the hot 240 line and ground and not worry about a hot (AC) chassis or anything nasty like that. I'll let you know how it works out after I get the windings stripped.It's tricky going,not much space to start off with and my right thumb is not cooperating like it should. Ever try and tie your shoes without using your thumbs? That's what I feel like.


As a side note,the oven xmfr is a good idea but the voltage would be far too high for 811A's,nearly 3000 volts with a capacitor input supply, unless it was rewound.

 

[linearone]

I'm going to try a microwave oven xmfr but in a differant application,as a filament xmfr.:blink: I was thinking that if it had a secondary voltage of somewhere around 2000 volts with 120 on the primary what would the voltage be if one were to switch the primary and secondary windings around. IOW apply 120 volts to the normal HV winding. The result was with 120 volts applied to the high voltage winding the now secondary winding had a voltage of 6.2 volts, a tad low for 572B's. I am in the process of stripping turns off of the HV winding which is now the primary and hope to get about 6.4 volts or so.A little high is easier to deal with than a little low.The other good thing is that the HV winding (primary now) has one side tied to the case and is grounded.Since I plan to run the amp off of the 240 volt line it means I can derive the 120 volts by tapping one side of the hot 240 line and ground and not worry about a hot (AC) chassis or anything nasty like that. I'll let you know how it works out after I get the windings stripped.It's tricky going,not much space to start off with and my right thumb is not cooperating like it should. Ever try and tie your shoes without using your thumbs? That's what I feel like.


As a side note,the oven xmfr is a good idea but the voltage would be far too high for 811A's,nearly 3000 volts with a capacitor input supply, unless it was rewound.

the amp is so small you could stick a tiny variac on the primary to drop the hv down. you must have one laying around right?

 

[Captain Kilowatt]

the amp is so small you could stick a tiny variac on the primary to drop the hv down. you must have one laying around right?

I'll let Radio Tech answer that one,it's his amp. Me personally, I would not want to be tied to a variac just to obtain the proper plate voltage. Besides think about that idea for a second. You say "a tiny variac". What do you call tiny? At full AC input to the four 811A's it will draw at least 10 amps from the 120 volt line. That's not what I would call a tiny variac. A monster no, but not a tiny one either.

 

[Radio Tech]

I'll let Radio Tech answer that one,it's his amp. Me personally, I would not want to be tied to a variac just to obtain the proper plate voltage. Besides think about that idea for a second. You say "a tiny variac". What do you call tiny? At full AC input to the four 811A's it will draw at least 10 amps from the 120 volt line. That's not what I would call a tiny variac. A monster no, but not a tiny one either.

Let's see,
It would take a full size variac to handle the PA. The problem is when you drop the voltage you are also going to drop in amperage. That would put you losing a third of your amperage if you were running 572B's correct? They can take the 2000 volt. But for 811 if would need to be around 15 to 1600 volts at 10 amp plus.

I played around with one of those transformers before with 4CX250b's. Went through 3 of them. I could never get them to hold. They seem to open up very easy. Maybe the RF? Not sure.

Anyway if I run the 572b tubes I am going to go with the 1800 volt transformer I have. Run it on 120. Switch the B- so zero voltage when idle. When keyed the supply should drop to a safe level for 572B's.

If I go with the 811A tube I am going to have to do something different.

 

[Radio Tech]

Just talked with a friend of mine. We were talking transformers. He has a 1800 volt and a 900 volt sitting around in his shop. Told me I could have them if I come get them!

Think I am going to

 

[Captain Kilowatt]

You can't beat that deal! the 900 would be great for the 811A's while the 1800 would make those 572B's stand up and holler.

 

[CDX-007]

- maybe if 2 or 3 microwave transformers were paralleled the lesser current draw from each would help them last...?

 

[Shockwave]

- maybe if 2 or 3 microwave transformers were paralleled the lesser current draw from each would help them last...?

Microwave oven transformers are nice and cheap but suffer from some problems. They are not intended to be powered on for long periods of time. Because of this corners are cut in terms of how many wraps of copper wire are used in them. The primaries do not have enough inductance to stay on for long. The AC output voltage is often around 2.5 KV and to much for 811A's after rectification.

I found and easy way around both of these issues. Buy used microwave oven transformers on eBay with 220 volt primaries and run then on 110 volts. The inductance within the windings is ample for 110 volts continuous use. You can also series both the primaries and secondaries of a matched pair and run a 3-500Z off 220 volts. Nice low cost plate supply.

 

[Radio Tech]

back to the top!

Well, after a bit more than a year later I am back to working on my homebrew amp. A few things have changed in the original plan. One is I will be using the 572b tubes now. So a little change in part location. Will start posting some pics of the progress as I move on with this.

Life has been very busy over the last year. At public job as well in the shop. Things are starting to slow a bit now so some time is there.

Hope every one is well.

 

[Radio Tech]

Spent this week looking at power supply designs.
Was looking in the shop and found a transformer from a pride DX 300. If memory servs me well these transformers are around 1800 to 1900 volts. Just can not remember what the amp rating is.

Also was looking at the power supply board for this amp. I have many Drake L4B power supply boards that will tuck neatly under the bottom of the chassis. So may use a pair of those. Save some bucks by using what I have.

 

[ElectronTubesRule]

I am also using Russian tubes and before I bought any I already knew I would need to slowly buy not just what I needed today but what I thought I would need tomorrow.

On top of that anyone that can build an amp with Russian tubes today can always recycle the amp's parts and install different tube 10,15,20 years from now. That is the beauty of building your own gear. While most have to use what they can buy from other's once you build a few amps and learn what you are doing you have the tools and knowledge to not be dependent on what other's are willing to sell you! The more you can do for yourself the more freedom you have.

Look at all the glass tubes and ceramic to metal tubes that Amperex, Eimac,GE and all the other company that no longer are avaialbe able. So compared to the Russian tube delima the once domestic tube market is terrible. We could prob. fill a book with all the tubes that have been used by A mature Radio operators that are no longer made in this country. Heck their is no source for new production American made 811A's or 572 or 833's let alone all the other's that are gone. Last I checked no 3-500Z's no 3/4 CX125,150,225,250,300,325,400 are not being made any more any place especially not in the USA.

So it is not a pretty picture no matter how you look at it. The tubes still available from Eimac for instance are limited as heck and expensive as heck!

 

[ElectronTubesRule]

I just bought 2 GI70B tubes for $14 and $14 to ship to the USA. That is $7 for each tube and $7 to ship each one. So for $28 total I have two 350W-450W tubes headed my way! That will make a grand total of 7 tubes of this model that I own. I would like to have at least 10 total. I plan on building one two tube model that is HF broad band and one that is a single tube 6meter mono-band amp.

If I need more power I will move to larger tubes not 3 or 4 tubes. So 9 tubes would allow me to build said amps and have the capacity to retube twice and have one left over.

The reason I am going with the cheaper GI70BT is that I already have 5 GI7BT's that came with silver plated aluminum coolers. So I do not need to have my spares with cooler's since I will use the cooler from any tube that dies to fit the far cheaper coolerless GI70 tube...... The "0" means the tube leaves the factory with out a cooler and they are about 1/2 the price of the ones complete with cooler's.

hard to argue $7 for a 350W-450W tube that is Mil Surp. and is metal to ceramic compared to $49-$99 for a single 100W sweep tube that we do not even know if it is truly NOS or lightly used or plain defective. The people I buy my Russian parts from test their tubes prior to shipping or they advertise such?!?

I think the same is true of any glass envelope tubes take the Russian GU13, 70 and 71 tubes no one knows how long they will hold out and we also know the Chinese versions are not that durable and that their quality control is hit and miss. The problem is they go for a lot more money because they fit in existing chassis. Bang for the buck the Military Surplus stuff is far better bang for the buck!