Standby Switch Wiring in Sentura II

[Leo_Gnardo]

By the way, where are you getting your electrolytic caps? I like to use axial to replace axial, when they're available. What good sources are there besides Digikey, Mouser, and Jameco? Digikey is not very price-friendly for low (repair) quantities. Mouser is a good choice. Jameco has a good selection of axials, but who knows what the quality of their private-labeled parts is? Weber has some Richey high-voltage caps.

Antique carries Illinois (I've had no problems), F&T (seems to be preferred by many home builders and boo-tikis), Atoms (at the best price you'll ever see), and if you like super reliability, Solen film caps rated for 630V use. Solens are not electrolytic and should last long past we're all deaf. They're kind of big, but if you don't ever want to worry about replacing them, and/or you need to handle that kind of voltage for a build or repair, Solens are the bees knees.

Magic Parts (supplier of Ruby Tubes) has a line of good quality house-brand caps. I've used hundreds and only one baddie - its label was on backwards. You have to be a business to purchase from them though, they're not looking for the hobbyist onesie-twosie buyer. I get the 22 or 47 uF 500V ones in 50-unit bags. Same for 100 or 220 uF 350V. If you know a shop that deals with Magic/Ruby you could ask them to get you some.

It's recently come to my attention that there's a very good line of USA-made caps, BMI, but I don't know of a source. Story is the company was founded @ 20 years ago by ex-Sprague employees.

[Enzo]

I agree that Magic Parts is a good company to work with, but you do have to have an account, they don't sell at retail.

ANtique is good - www.tubesandmore.com


And really, I think most places that sell guitar amp tubes online also sell caps and things.

[pdf64]

Is there an error on the schematic http://bmamps.com/Schematics/Sunn/Sunn_sentura2.pdf ?
I can't see a plate load for the 6AN8 pentode; just needs a link between the upper ends of the 12pF cap and 270k resistor.
Yes, of the 2 options for standby implementation, the Fender seems much better; the lowish value of reservoir cap may mitigate stress on the rectifier for the Sunn design.
Whichever, avoiding its use seems the best strategy, ie minimise stress / surge currents, thereby maximising the working life of affected components.

[patlaw]

There are many errors on the schematic. Here's a version in which I've corrected all that I've found so far.

[Leo_Gnardo]

And really, I think most places that sell guitar amp tubes online also sell caps and things.

True 'nuff. Have a look at Hoffman and Doug's Tubes, Mojo too. There's a few more besides I haven't bought from so can't say much but others have been satisfied, Tube Store for instance. There's also Parts Connexion in Canada, bound to be faster/cheaper shipping if you live in the great white north. They do ship quick to the USA too but you don't want to have to return anything across the border: major hassle. Pick one close to you if you don't want to wait long for shipping.

[patlaw]

What is the proper procedure for adjusting the bias on the 6550s on the Sentura II?

[pdf64]

A good guideline is to set the bias voltage such that the static power tube plate dissipation is in the range 50-70% of its limiting value.
For a 42 watt tube, 21-29 watts.
So, measure the static Vdc plate-cathode and plate or cathode current, and find the product.
If 1 ohm cathode current sensing resistors haven't been fitted, then the plate current can be determined by measuring the OT primary resistances from each plate to the CT, and then measure each plate - CT voltage under static conditions.
It's then beneficial to view the output waveform at various signal levels, to ensure correct operation, and similarly undertake a listening test.

[g1]

Digikey is not very price-friendly for low (repair) quantities. Mouser is a good choice.

Is it really that big a difference in price? I thought it was a very small difference, and I get the parts in 2 days instead of 2 weeks for the same shipping cost.

[Chuck H]

Is it really that big a difference in price? I thought it was a very small difference, and I get the parts in 2 days instead of 2 weeks for the same shipping cost.

Indeed! The shipping is the key. My last order with Mouser went better but up until about a year ago they use to charge silly high shipping for small orders.

[patlaw]

An individual shipping charge is not necessarily the problem, but when you have to make multiple small orders, as when you're doing repairs, the shipping charges can add up. Plus, there are some parts for which Digikey charges significantly more than Mouser for low quantities. The quantity pricing evens out. I understand the pain of dealing with small customers, be we have to have parts, too!

I don't know about you, but when I need a cap, I always buy two and sometimes buy 60, as I did today for 10uF 50v axial electrolytics. Antique had them for a quarter each, which is the best price I've seen in a while. Hopefully they're good caps. I use a lot of them by the hand full to refurbish Dorrough Discriminate Audio Processors. My shop is well stocked. I have every part - except the one I need. I started recapping a dbx 286a mic processor only to discover that I'd overlooked a 220uF 100V radial cap. It was part of the Mouser or Jameco order today.

I love Digikey. They never cease to amaze me with their inventory. They have some problems with their search engine (you can sometimes find part using the exact manufacturer's number that don't show up in their parametric search), but lately, Mouser has been beating them out a little.

While I have your attention, what's a good way to securely mount a 23mm axial cap (40uF at 600V) in my Sentura II? The top candidates so far are silicone, hot glue, perforated metal banding epoxied to the case, and a tie wrap. I'd love to find a CDE TH21 capacitor clip, but they're not available. While my amp will probably never go on the road again, you never can tell what could happen to it when I'm out of here. I try to do things the right way.

[Tom Phillips]

...what's a good way to securely mount a 23mm axial cap (40uF at 600V) in my Sentura II? The top candidates so far are silicone, hot glue, perforated metal banding epoxied to the case, and a tie wrap...

How about a cable clamp of the appropriate diameter?

[patlaw]

The Sentura II refurb is coming along. Parts are finally arriving. The first filter cap is now two 50uF/450v caps in series. The unloaded B+ is 604 volts. Loaded is 538. The picture shows how I mounted the caps. The scheme actually worked out pretty well since I didn't have to drill any holes.

The can cap was previously replaced and is good, but it's 525 volts. Unfortunately, Node A (immediately after the choke) is at 526 volts. I have a 40uF/600V cap that I can use there. I was hoping the voltage would be a little lower. I have a couple more of the terminal strips, so I hope that I can fit it in securely.

What's interesting is that I looked long and hard to find the right axial caps. Radials would have been easier to mount. Live and learn.

After doing all the work, when I first fired it up, it was dead. It turns out that the power switch is dying. What a pain. There's still more work to do, but it's getting there.

[Tom Phillips]

Nice workmanship on that cap mounting with the terminal strips.

[patlaw]

I assume it's better to hijack my own thread rather than create several new ones. If not, please let me know.

Three questions:

1. Where can I get a cap for the handle on my Sentura II? One is missing. See the picture.

2. Are the knobs the same as Fender knobs? Mine are getting ragged.

3. Is there a source for a power switch other than $17 on eBay?

[patlaw]

One of the four standoffs was missing from the tremelo board. In the spirit of WWCD ("What Would Conrad Do"), I found a 6-32 threaded nylon standoff by Keystone that allows the screw to remain screwed into the chassis while the nuts are removed and the board is removed. They're Mouser Part Number 534-1903B. They're pricey at 60 cents each, but you only need four.

Here's one of the standoffs beside one of the originals:



Here's a top view:



And here's what they look like in the chassis:



The only problem I had was the holes in the circuit board do not line up perfectly with the holes in the chassis. I had to open them up a hair in both the board and the chassis. I like it.

The amp is up and working. The next problem is that the treble pot seems to be bad. In fact, neither the treble, bass, nor contour control seems to have much effect on the sound. The treble pot is scratchy. When I worked it a few times, it started working better, but I'm going to replace all three since I have to take the face plate off to replace the power switch. Are the pots linear or audio taper?

What should I do with the accessory AC outlet on the back? Just disconnect it and leave it in place? I could put a three-wire receptacle there, but I'd have to use a knock out punch on the chassis to make a bigger hole, which I really don't want to do.

What's the consensus on the "death" cap? Take it out? It actually makes a difference in the level of buzzing in the amp.